Derry - singing session (Derry Traditional Singers' Circle) first Thurs of the month, Tinney's Bar off Strand Rd,, instrumental and singing every Tues night at Tinneys
Thurs. night sessions at The Derby Bar, Great James St.
Sun evening seeions at Sandinos, near the bus station, starting around 5pm
Ceili dancing (live music) and a bit of singing Monday nights 9-11 pm at A.O.H., Foyle St. (opposite bus station)
Set dancing class, Tuesday nights 8-10 pm, Pilots Row Community Centre, Rossville st (off William Street), 8-10 pm, £3.

I think there are regular sessions in Dungiven Co Derry

Inishowen County Donegal monthly singing sessions at McFeely's Bar, Clonmany - look up Inishowen Traditional Singers' Circle for more info.