Marie -- Two compacts might be cheaper and easier.  A comfortable car for six adults (AND Luggage) would be a VW Caravelle or similar and they tend to be PRICEY.  Even without luggage, the 7 passenger vehicles (typically a Ford Galaxy) use 'jump seats' for the two rear-most seats and they aren't particularly easy to access -- or comfortable to ride any distance in.

Are you leaving for SNN with them (and moving on -- to Dublin)?  Or, are you staying on, in Ballyshannon?  Enroute to SNN is Knock and several other interesting sites . . .

With a Ballyshannon base, you should have LOTS of options -- Slieve League Cliffs, Donegal town, Sligo, Beleek, Lower Lough Erne (Boa Island, Enniskillen, White Island, Devenish and numerous 'Plantation' Castles), and the Ulster-American Folk Park are all possibilities.

Around Sligo:  http://www.sligotourism.ie/attractions/type/top-attractions/   The Gleniff Horseshoe Drive:  http://www.benwiskincentre.com/driving-tour/   Rosses Point and Coney Island:  http://gostrandhill.com/explore/places-of-interest/coney-island/    Yeat's Grave: http://www.sligotourism.ie/attractions/w-b-yeats-grave-at-drumcliffe-graveyard/  

They have a GREAT teahouse!