A while back I reviewed a fairly newish book (published last year) titled Understanding Celtic Religion: Revisiting the Pagan Past, which I highly recommend. In the review I mentioned a particular article by Jacqueline Borsje titled "Celtic Spells and Counterspells," which is well worth a read.
I've just discovered that you can actually read the whole chapter online, so for those who're interested, have at it!
If that direct link doesn't work, try here. Just click on the pdf icon next to "Download" near the bottom of the page.
The article is mainly focused on Irish evidence, but it does bring in some comparative commentary, too, and the focus is on examining various examples of charms to try and untangle possible strands of pagan belief and practice. We begin (sort of, ish) with a discussion of the sugere mammellas or "nipple-sucking" episode that Patrick described in his Confessio, a rite of apparently pagan origin which he therefore refused to take part in. Evidence of possible pagan rites as described by Columba then follow, which leads into a discussion of the lorica ("breastplate") type prayers of protection.
Also included in the article is a discussion of a "spell" or charm attributed to St Brigit, which was meant to help a husband keep his wife (who didn't love him), the instructions for which include sprinkling of water over the marriage bed (which to me is suggestive of a saining ceremony of sorts). There's also a spell for impotence (with a translation given), which is rather ambiguous in nature – is it for causing or curing the problem? This in itself is pretty fascinating stuff, but it then leads into a discussion on the use of "words of power" – the use of seemingly gibberish or extremely obscure words or phrases to give an air of mysticalness etc. All in all you'll find a lot of food for thought here, both in terms of the kind of forms these charms could take, as well as what it can tell us about pre-Christian belief.