is í seo an tsráid is cúinge sa bhaile í.

Or:
tá sí seo ar an sráid is cúinge sa bhaile. (Tá with ar and the singular)

Thank you so much!

Is í an tsráid í is cúinge sa bhaile.
would be correct.

Thank you! You know, this whole subject of superlatives has lead me to research some more and find that I have a couple more questions, but I think I’ll ask as a new topic, since it’s technically a different topic.

Is í seo an tsráid is cúinge sa bhaile. (gan an dara í)

You must be right. I have seen few sentences with the emphasised pronoun in copula sentences. But looking in Ua Laoghaire's corpus you are right.

It is equivalent to sentences like Is ise ...:
Is ise an bhean.
Is í sin an bhean.

"í sin" is one of the two constituents, the other constituent is "an bhean" (or: "an tsráid ..." in your example).
(Which of them to be considered predicate and subject - opinions differ)
No third constituent could occur and no sub-predicate or sub-subject is needed.

In sentences with "Is í ... í", the first í is a sub-predicate, the last í being subject.

Well, if you're going according to O'Nolan's presentation (only one of the possible approaches), then in is ise an bhean, ise has to be the predicate. All presentations of the copula are confused, however, due to syntactical contamination centuries ago. O'Nolan argues that the the main thing is that the subject can never go next to the copula, which should point to the predicate. In is ise an bhean that principle is upheld. But in is í an bhean í, the problem is understanding why there is a subpredicate pronoun when an bhean is the predicate anyway, and not the subject. He argues the subpredicate crept into VpPS sentences by analogy with VpSP sentences (like 'sé is mian leis an Eaglais fearg Dé do mhaolú). In 'sé an namhaid an peaca, he states this could be interpreted two ways, VpPS or VpSP, but as the desired meaning is "the enemy (of man) is sin", it is VpSP, and in such sentences, which are a bit more unusual, you have to pronounce it more deliberately: 'sé (pause) an namhaid (pause) an peaca. And by analogy with that the subpredicate has crept into VpPS sentences too.

Rosie, the discussion becomes abstruse quickly. Let me set things out very simply. Please excuse me if this is already all sorted in your mind anyway.

V = the verb (the copula, is)
S = the subject (usually at the end)
P = the predicate (the information being given)
p = the subpredicate pronoun that has crept in over the centuries and is required before a definite noun

There are two main copula sentences.
1. Copula of classification, where you are saying "a X is a Y".
Is ainmhí capall - a horse is an animal.
This is VPS: verb is, predicate ainmhí, subject capall

2. Copula of identification, where you are saying "the X is the Y".
is é an múinteóir é - he is the teacher.
This is VpPS: verb is, subpredicate é, predicate an múinteóir, subject é
If you bear in mind that for historical reasons a definite noun like an múinteóir cannot go directly next to the copula, and requires a subpredicate pronoun, it should be plain sailing from then.

There are lots of disputes over the theoretical explanation of this, and various exceptions or rarer sentence types, but it's best just to focus on the explanation above, which explains the basic pattern.

Thank you both!
And thank you DJWebb, for the way you showed the formula for copular sentences…it was very helpful!