This sounds to me like it could be the Pangur Ban series by Fay Sampson.
I'm looking for a book I read around the mid 2000s.
I also read it in the mid 2000s!
I'm pretty sure it had the word "cat" in the title.
The first book in the series is Pangur Ban the White Cat.
It was about a girl who could talk to cats or understand them, and the kingdom she lived in.
I don't remember if Princess Finnglas had a particular ability to talk to cats, or if it was Pangur Ban who was special by being able to talk to humans, but certainly they could communicate with each other.
I'm sure her father died at the beginning, and he and her uncle had a terrible secret they and two other male characters had to find out while defending the kingdom or something like that.
In the second book of the series, Finnglas of the Horses (the only one I read), the protagonists find themselves in some sort of evil empire, and find the "terrible secret" that Finnglas's father the king is involved somehow (possibly something involving slave trade?) At the end of this book,
he dies, trapped under a beam in the collapse of a large building which caught fire. The fire was probably a result of what Finnglas and her male friend(s) were trying to do. I think the feel of it was that she wanted to save the kingdom even though its leader (her father) had done terrible things.
The book used a lot of Irish words, I specifically remember cac.
Although it's set in a fantasy world, there's definitely a Celtic/Irish feel to it.
One of the male characters used to be a slave or servant? Maybe. He had some of his fingers removed I'm sure, and the girl fell in love with him.
That would probably be Niall, the monk who befriends and travels with Finnglas and Pangur Ban. I don't recall them falling in love, but maybe that happens in the third book?
I’m think were three books to the series but I could be mistaken.
Yep: Pangur Ban, Finnglas of the Horses, and Finnglas and the Stones of Choosing.