Hi there,
I also loved her books. I suggest you read the Many Coloured Land series by Julian May. These are an enjoyable read, based on Celtic Mythology, but with an interesting twist... but well written - I've re read them many a time.
Can't think of any other Celtic ones at the moment that really stand out - although I am sure I've read a few! I also have a wonderful one - can't access at the moment, which is Celtic / Irish with a time travel theme to ancient ages.
Caseal Mor (not correct spelling) had a series out re Wanderers, which are specifically Irish... but I must admit I could never finish all of them (bought 3) and never could re read them - although others adore him (Aussie writer). I can't remember the titles- I ended up selling them 2nd hand for $1 each!
Hope that helps.
Sandra