Welcome to Michele's Forum!
Much depends on how you define "Castle Hotel". Most are actually based in Grand Manor Houses that may (or may NOT) incorporate a fortified tower house. Any of them would be more than "suitable" for two single women.
In the outskirts of Dublin, Clontarf Castle Hotel is one option:
www.clontarfcastle.ie/
Near Killarney and Dingle but actually in Tralee: Ballyseede Castle Hotel
www.ballyseedecastle.com/
In Cork, I've pleasant memories of a stay in Maryborough House
www.maryborough.com/offers
Near Galway, Abbeyglen Castle Hotel
www.originalirishhotels.com/hotels/abbeyglen-castle-hotel
Also in Galway (County) but actually closer to Clifden:
www.ballynahinch-castle.com/
Of course, if price is NO issue, there's always Ashford:
ashfordcastle.com/