I have a general and specific question regarding the copula.

1. When can you use the is ea construction, and what exactly does it mean?

2. "That is my daughter"
I believe that would be Is i m'inion í sin.
Of course that is just one way. Is it correct, and what are some other ways?

3. Combining the two questions above, would
M'inion is ea i seo be grammatically correct, and how exactly does it translate?

Just a change of word order for emphasis.
In English, you can change the sentence "I did the thing" into "The thing: I did it".
"It" taking the place of "the thing".

Same here: Is fear mé ("I am a man") -> Fear is ea mé. ("A man, I am it" or very literal "Man, is it I")
Ea ("it") taking the place of "fear".
Ea is only used in copula sentences. (so: not "An rud, rinne mé ea")
And ea can only substitute indefinite nouns, adverbs, phrases, whole sentences.
In any other case you use é or í (or iad).



Rather "Is í sin m'iníon" because demonstrative pronouns (í sin) usually come first.



No. Ea can only substitute indefinite noun phrases. But "m'iníon" is definite.
É, í, iad substitute definite nouns and noun phrases.
So it would be: M'iníon is í í seo. (But you wouldn't normally say so either.)