Yep, 'tis Irish. CIVITAS DVBLINIE mintmark on the reverse.
No idea of catalogue values - they're not technically "English" so they're not listed in the "Coins of England" catalogues and the only book I have that lists them is my first edition Friedberg (1962!) - but I can tell you I paid AU$144 for mine. It's been in my gallery since I acquired it:

Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis