Don't build a boat? The Irish will eventually try to land on mainland near the end so you need a boat to cut off their rolling army. They'll send about 6 full stacks on one turn once the war is declared.

I played as Saxons last time, my first game too, and I didn't have enough ships to cut off the Irish. Well, they were taking out rebels in the north but I just had to stop them. The other factions (Scots, Picts) were losing badly against rebels, turtling in little provinces up north. In result, I lost more than 6 provinces in midwest region. I did recover from the blitz after couple nasty battles but I ran out of time and lost the campaign lol.