copular forms-overview

lenition after the copula

• no lenition after present tense- and subjunctive-forms
• lenition after all forms in the preterite/conditional
  (esp. in Ulster t,d,s after ba is not lenited)

omission of the copular form is:

• The copular form is, preceding a pronoun or a noun, can be omitted :
  Is eisean an fear > Eisean an fear = He is the man
  In that case, an eventual subpredicate is omitted:
  Is é an fear atá ann > An fear atá ann = The man is here.
• in normal copular clauses, is cannot be omitted preceding adjectives:
  Is maith liom í = I don't like her *Maith liom é
  but in the usage for emphasis, one may omit it preceding an adjective:
  Is tuirseach atá sé > Tuirseach atá sé = Tired is he.
• other copular forms than is are never omitted.

miscellaneous and variants

• is is mostly pronounced [@s], with a broad [s], a slender [s'] only when preceding é/í/iad etc.
• with the pronouns é/í/iad as well as ea and éard (= é an rud) is contracts in in spoken Irish to [s'e:, s'i:, s'iad, s'a, s'e:rd]. Often this is also written as such: se, sí, siad, sea, séard (also with an apostrophe: 'sé, 'sí, 'siad, etc.): Is é an fear é > 'Sé an fear é = He's the man
• b'  mostly does not precede pronouns é, í, iad, ea :    e.g.: ba é = he was .
  Intead, a glide occurs in spoken Irish ([j] or [w]): ba é [baje:] or [bawe:]
• an older form is the spelling badh instead of ba, esp. in the conditional. (which explains the aforementioned glide: badh é)
• dob'/dob partially still in use in Munster(particle do + b') also preceding pronouns: e.g. dob'é / dob é = he was
• ar Connemara for an, ab Connemara for an preceding é, í, iad  e.g. ab é
• forms like ní ba, an mba, nach mba, go mba, a mba, mara mba are common in Connemara
  For the conditional, these forms reflect the original form. They are however replaced by those forms with -r in the standard, which originally are only the preterite forms.
• Instead of the subjunctive forms gura(b) / nára(b) in Connemara go mba / nár ba is used.
• forms of cha (chan, char, charbh) come up instead of ní in Ulster (and Scotland), whereby cha precedes adjectives, and chan precedes pronouns and nouns.
• Similarly, chan occurs in other forms ending in -n preceding nouns and pronouns, e.g. maran
• in Ulster also general forms like is é, ab é, gob é, nab é are in use (i.e. generally subpredicate é instead of é/í/iad, this é is independent of gender and number, partially also generalising in many uses, where it's lacking in other dialects, e.g.: Is é tinn a bhí sí = It's sick she was instead of Is tinn a bhí sí )
• combinations with prepositions demonstrate the indirect relative clause (e.g.: an fear lenar maith é = the man, who likes it, as the variant of the form: an fear ar maith leis é) When negating, only "an fear nach maith leis é" is possible.
• historical future forms of the copula (budh; also relative: bhus are now obsolete, but were at least used in written forms in days of yore.
• in Munster, one uses more often ná as the verbal particle and conjunction instead of nach. Though this is never used as the copular form, rather in that case always nach.

Seo, sin, siúd  as the copular forms

Seo, sin, siúd count as the copular forms in these examples.
They are forms of the demonstrative pronoun with the meaning "this is ", "that is ", "that one is "
All of these forms without s- are Connacht- (Cois Fhairrge-) Irish. I don't know if they exist in other dialects. This is why only the dialect specific forms (ab instead of an, marab instead of murab) are given.
As the standard, there are longer forms, in which the demonstrative pronoun is not seen as the copular form. Only the short form in the statement with "seo/sin/siúd" is also to be found in the standard.
 

• The subpredicate
  • A personal pronoun as the subpredicate appears in the short forms preceding definite nouns after the demonstrative pronoun, because this counts as the copular form. Seo é an fear. In Ulster it's omitted: Seo an fear = This is the man.
  • In the long form there is always a personal pronoun preceding the demonstrative pronoun. e.g.:
    Is é seo an fear = Seo é an fear = This is the man.
    Ab í sin an bhean = Ab'in í an bhean = She is the woman.
    Gurb é seo an fear = Gurb'eo é an fear = That this is the man.
• Why is the s in seo/sin/siúd omitted? Why does an apostrophe take its place?
  • An explanation is the consideration that, seo/sin/siúd are actually combinations of the copular form "is" and the little words   "eo/in/iúd", so that it must be "ab é is eo" , instead of "ab é seo" or even ab 'eo
    This view is, while etymologically wrong, (the s- in seo has nothing to do with the copula), is very practical (and stands in analogies to others, forms that are really constructed that way like is éard = séard, gurb éard, ab éard, etc.)
  • Another explanation is a lenition of the s: sheo, shin, shiúd. This h-sound is not spoken.
    Occasionally it is actually written that way: b'shin (b'in) = that was, b'sheo (b'eo) = this was, etc...