Actually, if we had unlimited province slots, I'd put 4 or 5 into Ireland (and at least 12 into Britain) - and say 14 into Iberia, and so on, and so on. It's just the reality of the situation: adding one somewhere means subtracting one from someplace else, and the map is finalized, at least in terms of province allocations: we really have been over every possible option, and aren't going to reopen the debate.


traditionally Ireland was divided into two regions during this period.
Well, not really. Have a look at Raftery's Pagan Celtic Ireland or A New History of Ireland. There are a number of ways that you could divide up Ireland in our period, but a straight North/South line is not one of them.

the archaeology at Emain Macha/Navan is far more substantial than at Tara Hill.
Again, not so much, or at least, they're kindof apples and oranges. The Navan site is thought to have been more ritualized, while the Tara site could actually have been used/inhabited as a residence/centre of power, but frankly there is so little real historical information about this period to help us interpret the archaeology that you could probably come up with a plausible scenario for just about anything.

That said, even if we had unlimited provinces, I would not be in favour of an Irish faction: there are just not enough real facts to work with.