Because the Irish people supported Britain.
The government was not so keen, hence they didn't join the war and claim the North as their reward.
Another example of the arbitrary assignment of neutral leanings, almost all of which can be questioned.
I like the idea of a set of cards, one for each neutral. One of these is picked up by each player on finishing his turn until all are collected. The player can reveal a card at any time to assign "pro" status to that neutral. If an unassigned neutral is attacked it joins the opposing alliance in the usual way.
Maybe a couple of "joker" cards can be included, which allow a player to stage a coup on a revealed but unoccupied neutral thus reversing the alignment.
Then make Spain or something pro-axis.