Is ... se, Is se...
When do you use "Is >...< se', and when do you use "Is se >...<"? I'm only starting to get these questions right because of repetition, not from understanding. I'm also not sure about the distinction between Is and Ta.
There are two perfectly accurate but linguistically complicated answers, so here's a simple answer instead:
She is A [thing] - Is [thing] í
She is THE [thing] - Is í an [thing] í
eg He is a teacher - Is múinteoir é
He is the teacher - Is é an múinteoir é
If by se you’d meant the masculine singular third-person pronoun sé, it’s not used in either case; it is its disjunct form é that is used in copular sentences with is. A summary of the word orders of copular sentences can be found at Gramadach na Gaeilge.
Sé could be used in a subordinate clause of a copular sentence, though, e.g. Is maith léi go bhfuil sé sásta (“She likes that he’s pleased”).
Regarding the distinction between is and bí (tá is a conjugation of bí ), if you’ve ever studied Spanish or Portuguese, it is similar (but not identical) to the distinction between ser and estar in those languages. If you’ve never studied either of those languages, the main distinction is that is is used to make statements of identification or classification, like “I am the walrus” or “I am a mammal”, and bí is used to make statements of existence or description, like “I am alive” or “I am fond of fish”.
It's usually helpful to give actual examples of sentences that you find confusing, because, as scilling points out, neither "Is >...< se' or "Is se >...<" are used in Irish.
Is é an t-innealtóir é - "He is the engineer"
Is innealtóir í - "She is an engineer"
The first of these is an identification phrase, with a definite predicate "the engineer". The first é is actually just a "subpredicate" used to separate the copula from a definite noun predicate - the é at the end is the actual pronoun "he".
In the second sentence, you have a classification sentence, with an indefinite predicate "an engineer". Here, there is only one pronoun, and it also goes at the end.
It can get a bit more complex than that, but that's a good starting point. Note that Ulster Irish doesn't follow these grammatical rules.