I just picked this game up, so I'm still learning some of the game mechanics. Currently i'm on the fourth generation of the Ua Brian dynasty, and have all of Ireland under my control except for the county of Ulster, which belongs to the Scottish crown. Somehow the Scots ended up with a claim over the county of Oriel, although I really don't recall getting a message about them fabricating a claim, and I didn't marry anyone off to them that would have had a claim to it. Long story short, I can't avoid a war with them. I have 400 gold to my name as I just got done placating all of my vassals, who were still seething at the tyranny of my last king. My only reliable Ally is Brittany. Scotland, on the other hand, can call on England and Norway, and they both readily answer the call.
Result? Tens of thousand of men landing on my shores that I simply have no way to stop. Is this pretty much game over for me, here? I can hire mercenaries to barely keep the pace, but the battles take so long that I run out of money. The last time I tried this I ended up in the red and had to dismiss my mercs just as the second wave hit my shores.