"These are our heads."
Translation:Is iad seo ár gcinn.
It rejects Seo iad ár gcinn.
A common way to introduce someone to a third party is to say for example Seo í Úna
Two years later, but :) Is iad seo ár gcinn. seems the correct way to say it, and not only in Ulster Irish. This was what I answered based on what I had read on GnaG. It's again an example of copular sentence where the subject comes before the predicate because the subject is a demonstrative pronoun.
edit : The other sentence I was referring to is : Is é an fear sin a múinteoir. (here) In that example, the subject was not a demonstrative pronoun only, but held one, and in that case the subject also comes before the predicate.
In addition to being explained in the "Exceptions to the PSO rule" (link in the other comment thread), it's explained there, under "wenn das Subjekt ein Demonstrativpronomen ist bzw. führt.".
You could get away with Seo ár gcinn, but Sin ár gcinn would mean "Those are our heads".
When a part of the Sentence is elided like this (the copula is really there, just not spoken), Duolingo usually only teaches the un-elided form. It's a lot easier to learn to elide something as you get better than it is learn to remove the elision.