Unfortunately there's a good deal of backwards projection with how Ireland is portrayed in EU4. The reason Ireland is as small as it is today is due to centuries of British (mis)rule, with laws banning the Irish from walled towns, repeated genocides, lots of bloody rebellions, and the siphoning off of taxes and resources to England.
There's none of that in game, though. It's small and weak from the start, and there's little you can do to change it, short of wasting mana on development, even if you manage to unite Ireland and keep it independent (thus avoiding the worst of British rule).
Ideally, it would have maybe 3-4 more provinces, a trade node in the Irish Sea, a slight bump in development (~10), and more accurate terrain (Meath should be farmland, Ulster - hills, Connacht - highland, Leinster & Munster - woods or grasslands). That way developing the provinces will be a bit more feasible if you're playing in Ireland, but they'll stay poor and the trade value will be siphoned off if England conquers it.
Events for some of the Irish rebellions would be nice, and a more fleshed out 'Wars of the Three Kingdoms'. And more generally, it would be nice if putting down rebellions sometimes reduced development in provinces.