Visit http://mysite.wanadoo-members.co.uk/Kingthings/fonts/fonts.htm and scroll down the page to find Kelltika. It is based on the book of Kells. The rest of the fonts aren't Celtic, but they're very nice fonts nevertheless. Enjoy!
Sorry; you'll have to copy and paste the link to use it. My HTML coding isn't all I wish it was, and my attempt to include a link messed up somehow. The time limit for editing rules out any attempt to look up what I did wrong so I can fix it.