Here is a decent explanation. It's been covered in a few lessons: http://en.wikipedia.org/wiki/Irish_syntax#The_copula_is
It has taken me until now - four years after this message - to find Tips and Notes on the computer. When you hit a lesson set (say, Basics 1) a rectangle opens below it that tells you which set you're on and how many still to go before you reach level 5.
At the top right of that rectangle is a lightbulb icon. THAT's where I found the Tips and Hints! I could have screamed...
Scroll down on this page for the Tips and Notes. I use the computer and the mobile phone. If you have wifi, you can access the internet page that the computer users see. https://www.duolingo.com/skill/ga/Basics-1
Rumoured, avatars are optional so you cannot assume someone is on mobile by that.
ataltane, the Tips & notes appear on the top left on my computer lesson pages.
On page 9 at this site, they call é, í, íad emphatic forms which is a better term to use to not confuse this form with an object in this construction where there is no object. So, I would rather think of it as the emphatic form can also be used as an object? if that is so?
With real verbs (like tá or itheann, but not with the copula is), it works like this:
sé = he; é = him
sí = she; í = her
siad = they; iad = they
Two potentially confusing things:
The other personal pronouns have only one form (mé = I or me; tú = you)—which is just like the English word 'you'.
The copula pattern (with 'is') always takes the shorter form (é, í, iad), if it exists, though an English speaker might expect the longer form (sé, sí, siad).
Would it have something to do with the fact that this is copula ends with a letter "s" and those persons sé, sí and siad begin also with a letter "s"? I immediately made that association.
I also think that only sé, sí, and siad have their initial "s" dropped — while there are other grammatical persons with an initial "s" — because they are third persons, and in many languages third persons have particularities in their behaviours. At least, that is how I understood it.