If you are getting the ferry from Belfast you could always head north & visit the coast of Northern Ireland - it's beautiful & there is lots to do, for example the Giant's Causeway, Carrick-a-Rede Rope Bridge, Carrickfergus Castle to name but a few.
Then you could cross the border into Donegal, which is wild & wonderful, with fabulous beaches.
Westport is lovely, and Achill Island nearby is truly stunning.
The west coast of County Clare is gorgeous, and is not too far from Galway city which has lots to do.
Kerry & Cork in the south west are fabulous for their beaches & scenery, but it's a long drive from Cavan.
Some friends of ours stayed in a self-catering park in Cavan which had log cabins to stay in - unfortunately I can't remember the name of it, but they loved it.
I know I probably sound like a tourist brochure, but I moved here with Irish DH several years ago & I love the place. If you get good weather, there is nowhere like Ireland for a holiday - unfortunately it does rain a lot!