Cail is right in that there are so many Celtic fonts out there and to make sure everyone has it installed is a pain. I would go for one that is freeware (ie doesn't cost anything to get it). There is a well known site called: www.dafont.com

This site has many historical fonts on it including the usual ones that most people have. It's the encyclopedia of freeware fonts! *G* I'd also choose one that is legible at any size as people have different screen sizes and also read in different zoom sizes. Some of the more elaborate fonts tend to squash and blur and you can't read them at smaller sizes.

This is the Celtic category of the above site: http://www.dafont.com/theme.php?cat=403

I would recommend PR Unical, Mael, Seven Swordsmen and possibly Stonehenge out of all the fonts in this category. I have used all four with excellent results. Seven Swordsmen can be a bit heavy for some graphics so if you want a whole page in this font, then it's not the right font for you. Stonehenge can be a bit lightweight for the same reason. PR Unical is probably the best. This is a hand drawn font by Peter Rempel and his work is fab!

Try any of them and see which one suits/looks better. If you choose a font that is not exactly standard, then put a link at the bottom of the page so people can download it if they want to.

:D