I hunted down a reference for the hair liming info I posted yesterday. Stefan's Florilegium is a collection of useful information for historical re-enactors. Here is a page that has a wealth of great stuff on hairstyles of the medieval and renaissance periods, with a few posts about the ancient Celts woven in.

Scroll about 2/3 of the way down to see the information on Hair Liming. I am not endorsing this, by the way. If you want the Celtic warrior look, there are safer ways to do it besides liming that will still give you an authentic appearance without sacrificing your hair.