The Ferrycarrigh is fab and well worth it. The shannon is great we did a weeks on it a few years ago, but I wouldnt feel save with young kids, we used Derg line cruisers and they were wonderfull boat.
I was going to say Westport too, the Breafey house just a few miles from Westport, its a lynch hotel and do great offers and they have a great spa, some offers you get a free treatment. Dont think the Park or house will be open at this time of the year, we went in March and it was closed
HTH