Regular Singular Points

I was reading a blog entry, from someone who recently moved to Paris, pointing out that the hot water taps in France are marked with the letter ‘C’ and it reminded me of a funny thing about the Irish language.

Here’s the setup: you’re a foreigner in Ireland and you’ve had 4 pints of Guinness and find yourself in need of the loo.  Someone points you down a dark hallway and there are two doors, one marked “Fir” and one marked “Mná”.  You wonder, briefly, whether you’re latently dyslexic, but can’t figure out how to reconfigure “fir” so that it makes any sense at all (remember, four pints are involved).  Not that it matters in Ireland, but which door do you choose?