I'm not sure of their UK concert dates either, but if you can get over to Donegal, Northwest Ireland, for the last week or thereabouts, of the year, there is the Frankie Kennedy Winter Music school in Bunbeg. You won't just see Altan in concert but you'll be having a pint or two with them and maybe even a session.

Oh, you can also learn to play Donegal style if you want.

Hope this is of use.

Slan

Laoise