Regarding flights, you should check out Aer Arann or one of the smaller carriers - I think they might fly from Glasgow to Carrickfin (small airport in Co Donegal).
The ferry option means you would probably have to rely on public transport to travel from Larne to Derry (perhaps changing at Belfast) and then on to Donegal.
The Airporter bus service operates a frequent service from the international airports at Belfast and Belfast City and this would bring you to Derry also.
N Ireland - Translink (also known as Ulsterbus)
Rep Ireland - Lough Swilly (also Bus Eireann).
Hope this helps - feel free to ask for more info.