I think you are right, Lasair, about the Celts using urine mixed with the lime to calm down the chemical effect it has on us. I know straight lime is pretty painful, as I got it on me once. Apparently it is used to make concrete also, and I was moving bags of it and one broke on me. The lime actually ate away some threads on my clothes, so I suppose it is pretty serious stuff.

I know that the Celts, and most specifically, the Celt-Iberians would use urine to bathe in if we are to believe Diodorus of Sicily. They would also use it to scrub their teeth with as well. Though I would only take his account with a grain of salt, because much of what he writes about the CeltIberians is taken from Strabo and Polybius. He may have gotten his information confused. Here is a cross post by Fedelm Cruithni at the group Alba on on Gaulish Lime Wash.

Although they might not have been too far off, as I have also read that English people into the 17th century would preserve their wheat and barley in stale urine to keep the birds away from it. If that is the case, then the term, "This beer tastes like urine" isn't too far off.

As far as following any of the ancient recipes of adding urine to lime to put in my hair, I think I'll pass.

Here is a little page I found on Lime in the Hair.