As you certainly know, the key difference in the language is that between is and tá, also called “the copula” and “the substantive verb”. However, so as to speak and write good Irish, you need to grasp and master it.
As we should know by now, you can’t use tá to link two nouns, if you don’t put something between them. However, there is one important exception. You are allowed and indeed encouraged to use tá if you are telling how long, how big, how old etc. something or someone is. Thus, look at this:
Tá Seán trí bliana déag d’aois. “Seán is thirteen years old.”
There is no prepositional construction, no nothing, between Seán and trí bliana déag. Seán is thirteen years and you use tá to tell it.
Note that the trí bliana déag is followed by d’aois, i.e. “of age”. You either use ar or de before the abstract noun in this construction. It is a question of idiom whether it is ar or de. For instance, with fad and leithead it is ar (which does not lenite here: ar fad).
The formula is:
TÁ + SUBJECT + NUMBER + UNIT OF MEASUREMENT + DE/AR + ABSTRACT NOUN (QUALITY MEASURED).
There is a certain copula construction which most grammars for learners do not concentrate enough upon, and that is the one where we have:
COPULA + NOUN/ADJECTIVE + PREPOSITION DO OR LE
Usual examples of this constructions are:
is dóigh liom “it is my opinion, I think”
is maith liom “I like”
is fearr liom “I like better, I prefer”
is maith dom “it is good for me”
is fearr dom “it is better for me”
is féidir liom “I can” (I am able to, I feel like doing it etc.)
is féidir dom “I can (there is no reason why not)”
Note that there is an important difference between le and do in these constructions: where both can be used with a certain noun/adjective following is, the le construction refers to something subjective or personal, but the do construction refers to objective external circumstances. I found this (here standardized) example in Seán Bán Mac Meanman’s collected works:
Ní haithríoch leat, ach is aithríoch duit!
The dictionary translates the adjective aithríoch as “penitent”, but I’d say this example means “you have no regrets (i.e. you don’t feel so, subjectively), but you shoud have (i.e. you have objective reasons to feel regret)!” This is as good an illustration of the difference between le and do in this construction as any.