Relative clauses are subordinate clauses, which refer back to a word (antecedent) in the primary clause. In Irish, they are introduced by a relative particle, more rarely by a relative pronoun. The relative particle is usually not a pronoun. Is links only an antecedent and a relative clause (thus it is a conjunction.)

In Irish, there is a distinction between direct and indirect relative clauses. In direct relative clauses, the antecedent is a direct subject or object. In indirect relative clauses, a there usually appears a further pronoun which has the function of an object, and which itself refers back to the antecedent.

For syntactic reasons, relative clauses are used by far more frequently than in English.

The direct relative clause (an clásal coibhneasta díreach)

form: One uses the direct relative particle (mír chobhneasta dhíreach)

• affirmative clause: present et al..: a (Lenition), past: a (Lenition)
• negative clause: present et al.: nach (eclipsis), Past: nár (>Lenition) (in Munster ná instead of nach)

Characteristics:

• In both the present and future special relative forms of the verb are used (with s-endings: Present: -(e)a(nn)s, Future: -f(e)as or. -ós)
  e.g.: bíonn > a bhíos (also a bhíonns), beidh > a bheas, feiceann > a fheiceas (also a fheiceanns), feicfidh > a fheicfeas, éireoidh > a éireos
  These relative forms are widely common, but not part of the standard.
• An exception is tá, which is always “merged” with a to form atá. Likewise adeir, adúirt, whicare mostly written todays as a deir, a dúirt = (says, said).

Uses:

• as an attributive subordinate clause referring to an antecedent. The antecedent is the subject or direct object of the relative clause.
  e.g.: an fear a rinne sin = the man who did that; an rud a rinne sé = the thing that he did
  position of the relative clause:
  • attributive relative clauses mostly stand directly after their antecedents, in the sentence structure the antecedent and the relative clause often move to a place that allows for this, usually at the send of the sentence::
    e.g.: Tháinig isteach i dteach tábhairne an fear a bhí ina chónaí sa Spidéal = Into the pub came the man who lived in Spiddal.
  • or at the beginning of the sentence, if it is the subject (with a pronoun in its place after the verb)
    e.g.: An fear a bhí ina chónaí i Spidéal, tháinig sé isteach i dteach tábhairne = The man who lived in Spiddal came into the pub
  • also, the relative clause can be separated from the antecedent:
    e.g.: Tháinig an fear isteach i dteach tábhairne a bhí ina chónaí i Spidéal = The man came into the pub, the one who lived in Spiddal
• relative clauses that refer to an entire sentence of sentence subset begin with rud a (in English “which"):
  e.g.: Dúirt sé liom an leabhar a cheannach, rud a rinne mé = He told me to buy the book, which I did.
• to get around the PSO word order, sentence elements can be fronted, and the rest of the sentence follows as a direct relative construction (as a gap clause, see emphatic sentence position). In this case there is no proper relative clause: the relative clause serves here only syntactic functions (the so-called independent relative clause, clásal coibhneasta leithleach)
  e.g. Is ag léamh a bhí mé = I was reading. lit.: "It-is at reading that was I". [The emphasis would largely be conveyed in English by a stronger intonation on “reading.”]
• after question words (cé, cad, ceard, conas, etc.) a direct relative clause always follows. e.g.: Cé a bhí ann? = Wer war da?
  (if cé, cad are linked with prepositions (cé air, cad chuige, etc), an indirect relative clause follows! Likewise after cén fáth, cén chaoi and a few others.)
• after an té (the one), cibé, pé (whoever)
  e.g.: an té a chonaic sin = the one who saw that; cibé ar bith a chonaic sin = whoever saw that
• after the conjunctions nuair, mar
  e.g.: nuair a bhí mé anseo = when I was here; mar a bhí sí = as she was
• together with chomh (+ adjective) et al. with agus (in the sense of “As….as") :
  e.g.: chomh luath agus a bhí sé ann = as soon as he was therer; an oiread agus a fuair sé = as much as he got)
• after the abstract_noun_of degree(see there) e.g. dá fheabhas a rinne sé é = as well as he did it.

Characteristics of the syntax:

If a sentence with a go/nach subordinate clause is converted into a direct relative clause, the go/nach subordinate clause is also converted into a direct relative caluse. The previous primary clause is converted into a module clause (clásal idiraisnéiseach) as in our example “whom I think.” It usually involves a verb of saying or thinking.

The indirect relative clause (an clásal coibhneasta indíreach)

form: The indirect relative particle (mír choibhneasta indíreach) is used

• positive: Present et al.: a (Eclipsis), Past:ar (Lenition) (in Munster go/gur in stread of a/ar)
• negative: Present et al.: nach (Eclipsis), Past: nár (Lenition) (in Munster ná instead of nach)

Use:  The indirect relative clause is either

• prepositional (e.g. Eng. on which),
• genitive (e.g. Eng whose)
• adverbial (Eng. when, where, how),
• used with a back-referencing prepositional pronoun as an accusative object.

A back-referencing pronoun (forainm iartheachtach) appears both in prepositional use (mostly as a prepositional pronoun) and in genitive use (as a possessive pronoun). Is can also appear as a prepositional pronoun in the form of an accusative object (without this pronoun a direct relative clause it necessary)

This back-referencing pronoun represents the antecedent (réamhtheachtaí) as an object. The antecedent is only more indirectly the object of the sentence (represented by the pronoun). In the direct relative clause however, the antecedent is always the direct object or the subject of the sentence.

I. prepositional use (on which.... / in which... / etc.)

1. Preposition with personal pronoun at the end of the clause
 

The personal pronoun is here the back-referencing pronoun.
Combinations of prepositions and personal pronouns are the most common occurences. (Prepositional_Pronouns).

2. Preposition before the relative pronoun
 

A preposition can introduce the relative clause. [Learners of English who remember being scolded not end sentences with prepositions will recognize this as the “proper” or “formal” form in English. In Irish, like in English, the previous form, with the preposition at the end, is more common and more natural in living speech – DN]

This variant is only encountered with a few prepositions (ar, as, do, i, le) and
it is only possible in affirmative sentences, not in negative ones: e.g.:

An bord ar a bhfuil an forc = the table on which the fork is (affirmative) but:
An bord nach bhfuil an forc air. = the table that the fork is not on (negative)

In this case the relative particle is a genuine relative pronoun, the so-called back-referencing relative pronoun (forainm coibhneasta iartheachtach).
The back-referencing relative pronoun requires certain combinations of the prepsitions and the relative pronoun a / ar.
Of these, the most frequently appearing forms are lena(r), dá(r), ina(r)
 

II. genitive use (whose...)

Here the possessive pronoun is the back-referencing pronoun. It must refer back to the antecedent of the relative clause. [1]

III. adverbial use

The antecedent must be a general term of time / place / reason / and way. A direct relative clause is equally possible here.
e.g.: áit = place, slí = way, caoi = manner, dóigh, fáth = reason, am =time, lá = day, oíche = night, etc. (uair however is not usual)
The antecedent can also occur in questions, e.g.: cén chaoi, cén fáth (Cén chaoi a bhfuil tú? = How are you?)

These uses are called adverbial, since the relative particle here replaces the relative adverbs when, where, who, how, etc.
In English this is very common:

e.g.: an áit a bhfuil mé sásta = the place where I'm satisfied
       an fáth a bhfuil mé sásta = the reason why I'm satisfied
       an t-am a bhfuil mé sásta = the time when I'm satisfied
       an tslí ar fhoghlaim mé Gaeilge = the way how I learned Irish

In principle, this adverbial relationship can be convered into a prepositional relationship (the place in which, the time at which, etc.). This means that a back-referencing pronoun would be conceivable.
(e.g. an áit a bhfuil mé ann / an áit ina bhfuil mé = the place that I am in).

Actually, the indirect relative pronoun with the preposition “i” ina = in which was was reduced in the course of time to the modern a.
In earlier times, i (in the sense of “in which,” as an abbreviation of ina) was written here.
(an áit ina bhfuil mé > an áit i bhfuil mé. today: an áit a bhfuil mé).
 

IV. use with a back-referencing personal pronoun as accusative object.

Principally, when the antecedent is an accusative object, a direct relative clause is used.
However, direct relative clauses like "An fear a bhuail mé.”are ambiguous.
There is no way to distinguish whether the antecendet is the subject (”the man who hit me”) or the object (“the man whom I hit”).

In the latter case, an unambiguous sentence structure with the indirect relative particle is possible.
 

The back-referencing pronoun (usually é, í, iad) is important.
Without it, a direct relative clause would be necessary.

Special properties of the indirect relative clause

A go/nach-subordinating clause my be converted into an indirect relative clause if one turns the main clause into a relative clause, similarly to direct relative clauses. A prerequisite for this is a potential back-referencing pronoun, or the creation of such (in the example below: air = upon it)
The main clause becomes an inserted clause (clásal idiraisnéiseach), in the example: "of which I thought ".
According to the Bráithre Críostaí, the inserted clause is a direct relative clause: "a shíl mé"
According to Ó Siadhail it is an indirect relative clause: "ar shíl mé"
 

The conversion of the go/nach-subordinating clause is not required. Also sentences like this are possible:
An bhfuil an forc ar an mbord ar shíl mé go raibh sé air? = Is the fork in the table, of which I thought, that it was on?
The indirect relative clause (ar shíl mé ...) is comprised here as well of the prepositional pronoun air, referring back to bord, so it is a back-referencing pronoun.

Hence, three possibilities come about of this sentence:
An bhfuil an forc ar an mbord a shíl mé a raibh sé air? (dir. + indir. relative clause)
An bhfuil an forc ar an mbord ar shíl mé a raibh sé air? (indir. + indir. relative clause)
An bhfuil an forc ar an mbord ar shíl mé go raibh sé air? (indir. relative clause + go-subordinating clause)

The indirect neg. relative clause identical in form to the direct neg. relative clause (as well nach/nár), with the exception of a naturally occurring back-referencing pronouns (in the examples in bold)
A preposition is not possible as the introducer of the relative clause.
 

Clauses with the generalising relative pronoun a

In this instance, a / ar means:  "everything which " or "all that"
even this a requires eclipsis, ar lenites! This generalising relative pronoun (forainm coibhneasta réamhtheachtach) looks the same as the indirect relative particle, but the antecedent is missing!
The missing antecedent (réamhtheachtaí) is expressed by the relative pronoun itself.
Hence the term "réamhtheachtach"
It can be the object (sin a bhfaca mé = that's all that I saw ), as well as the subject (sin a bhfuil anseo = that's all that's here) of the relative clause.
Sin a bhfuil is short and sweet for: That's all.

With the help of the preposition de (= of) one can closer modify what is meant by "all".

emphasis of a selection with dá (= of all that which)

This dá is a combination of de (= of) and the general relative pronoun a (all that).

use of the form dá:

• often with superlatives
  e.g.: An fear is airde dá bhfaca mé riamh = the biggest man that I have ever seen. (lit: "...of-all, that saw I ever ")

  after gach + noun and similar expressions:
  e.g.: gach ní dá mb'fhéidir = everything within possibility (lit.: "every thing of-all, that are possible ")
  after timepoints-/locations, also in a rather adverbial sense:
  e.g.: gach uair dá raibh mé ann = everytime I was there (lit.: "every time of-all, that was I in-it ")
  gach áit dá mbím = everywhere I am (lit.: "[at] every place of-all, [at] which I am")

  after iomlán = everything and other expressions:
  e.g.: Iomlán dá bhfaca mé riamh = All I ever saw (lit.: "all of all, that saw I ever ")

• after dá + abstract noun:
  e.g.: dá óige dá bhfuil sé = however young he is (lit.: "of-his youth, of-all that is he ")

negation of a = all with an méid

nach is here a normal relative particle and not a pronoun, which is replaced by an méid. The whole clause is a normal direct relative clause.

negation of a = everything with diabhal (= devil)

a is again the generalising relative pronoun, which is negated by diabhal.
diabhal a means then nothing.
With ach an only-clause is created: Diabhal a bhfuil ann ach leabhartha = Only books are there.

[1] The genitive use comes from the prepositional use (selectively, the prepositions ag, i, do)
an fear ag a bhfuil a mhac ann = the man whose son is there
an fear ina bhfuil a mhac ann = the man whose son is there
an fear dá bhfuil a mhac ann = the man whose son is there
(ag a > 'ga > gha > a, ina > 'na > a, dá > dhá > a)
ag a > 'ga > go - also the reason why in Munster, one uses go instead of a