In Irish there are 2 words, which can be translated into English with "to be ".
While the copula "is"only serves to conjoin subject and predicate, the verb bí is predicative itself :

Bí expresses an action of existance is a (named or unnamed) state of being: Táim = I am (I exist, there is a "me"). It is then a verb of the state of being, of existance.
This is why it is mostly used with prepositions of location, in simplest form with i (= in) or its 3rd person masculine conjugation ann (= in him/in it/therein, extended "there/here"). (comp. a state of being in which one is) e.g.: Tá daoine ann = People exist / There are people here.
Existance alone may by freely expressed without an i/ ann , e.g. Níl aon Dia ach tú = There is no God aside from you.

Also other states/conditions can be described (e.g. the state of being of standing, sitting, living (see state of being verbs), the state of being in a profession/practicing a trade, etc.). Furthermore, it is used with predicative adjectives.

Examples:

This existance/state of being/location description is the base meaning of bí.
The extent of existance or the magnitude is expressed without a preposition. i.e. statistics such as height, weight, age, time and the like is done by usgin bí + magnitude:
e.g.: Tá sí seacht mbliana d'aois = She is 7 years old; Tá sí naoi gcloch meáchain = She is 9 stones (=57 kg) heavy; Tá sé ceathrú chun a trí = It is quarter to three.

The modern uses as the auxilliary verb (e.g. in the progressive) are secondary, but more or less contain the sense of a temporally limited state of being. In the progressive, the temporal limitation (to that moment) is especially emphasized.
Tá sé ag ól = He is drinking (right now at this moment)

Predicative adjectives are mostly used with bí today, and the temporal limitation moves into the backgound, because a state of being is being described.
Because bí itself is predicative, its use as a predicative adjective is grammatically problematic, and earlier it was only considered possible to make the adjectives into adverbs (with go). This has remained so only by a few subjective adjectives of assessment (Tá sé go dona = It is bad ). Otherwise, predicative adjectives are used today (or it's the case that they are indeed adverbs, whereby go is omitted): Tá sé mór = He is big
The copula can also (less common today, earlier always) be used with adjectives. Today, this is limited to a certain few expressions. In this case, all signs of a temporal limitation are lacking, and it is moreso the permanent intrinsic quality being described. All in all, the differenced in meaning between bí and is with adjectives is rather small. The copular clause emphasizes more the adjective, and is therefore often interjectory ("He is big!")

With the construction i + possessive pronoun + noun , bí can form a clause with a similar meaning as that with the copula. (see classifications- and identification clauses without the copula)
So, instead of an actual classification/identification (with the copula), there follows a description of a state of being (with bí). e.g.:

The copular clause gives a rather (mostly permanent) assigment "he = doctor ", while the clause with bí shows that a (mostly temporary) relationship exists, which was just achieved or could be reversed, and therefore is more a state of being (he is now a doctor, he works as a doctor, but will not always do so)
e.g. Tá sé ina fhear anois = Now he is a man (lit.: "is he in-his [state of being of] [being]a man now ". Because before that he was a child)
With the use of finite temporal adverbs, e.g. anois = now , the construction with the copula also becomes possible, e.g. Is fear é anois = Now, he is a man ), as well as the use of bí with infinite temporal adverbs, e.g.: Bhí sé ina amadáin riamh = He has always been an idiot. Through this, the above statement receives a permanent vs. temporal relativity. The main difference of the sentences is the very classification through the copula, where bí states that it's a case of state of being being described.
e.g.: Is fear é = He is a man : He is classified as a man.
e.g.: Tá sé ina fhear = He is a man : He is in a state of being, in which he posesses the qualities of a man.

Different words for "being", and special copulae exist in other languages as well: e.g. in Spanish "estar" and "es"
(notice the non-coincidental similarity between "tá" and "estar" as well as between "is" and "es"!)
The form tá derives from the indoeurop. root staro = to stand to (underlining the verbal base meaning as the German "Zu-standes")