There's a decent amount of public transport between Derry and Buncrana, in fairness. Not as frequent on Sundays though. Definitely handier than from Letterkenny.
http://journeyplanner.translink.co.u...181_y15_TP.pdf
http://www.mcgonaglebushire.com/time-table/
Worst case scenario, you could get a taxi (€25ish) or if we're driving, could give you a lift.