I am trying to remember a book I read, within the last 10 years, and I don't think it was published too long before I read it.
Here's what I remember.
It follows a Norman knight sometime after the invasion of Ireland. He has been sent their either because he inherited some land there or as a form of punishment.
There is a very strong Celtic mythology presence, Redcaps play a very prominent role as antagonists, killing quite a lot of people.
The Morrigan is also in it she is very manipulative and trying to start a war. She can turn into a crow as per her mythology.
There are various other celtic gods/spirits etc at work throughout the book, both trying to help and hinder the knight.
It may be part of a series though I've only read the first one if it is.