Paploo,
The copula has deep and profound mysteries. All I can say is practice makes perfect. A sub-page of the link below gives some detailed information on the syntax.
http://nualeargais.ie/gnag/kopul1.htm Under "The classifactorial clause (an abairt aicme)":
Dochtúir is ea an fear = lit. "A doctor, is it the man"
The 2nd variation of the [classifactorial clause] is actually a form of emphasis with a trailing copular clause (like a left shift):
Although in Munster, this is the general common form of the classifactorial clause. But, it will only occur in the affirmative clauses.
A common form of emphasis in Connacht and Donegal would be a relative clause with bí and the preposition i (as an inversion of the clause Tá sí ina dochtúir, see classifactorial clauses without the copula ):
Dochtúir atá inti = She is a doctor.
Rud a bhí ann nárbh fhurasta a dhéanamh = That is something, that is not easy to do.