My wife and I consider Donegal and Connemara to be the two prettiest areas in Ireland, and part of their charm is the fact that they're well off the usual tourist track.
From Knock you have some great options. Sligo is a pretty little town, and nearby is the highest concentration of megalithic tombs in Ireland - if not all of Europe. Getting to them requires a bit of hiking however, so be prepared for that.
Clifden would be a good base for exploring Connemara. The Sky Road just west of town is beautiful. There's also a terrific loop drive that connects the National Park with it's scenic hike up Diamond Hill, Kylemore Abbey and the pretty village of Cong. Nearby Ashford Castle has a Hawk Walk that many here rave about, and you don't have to stay there to enjoy it. There are also easy day trips out to Inishmore on the Aran Islands as well as the shorter trip out to Inishbofin.
Donegal is Ireland's Wild West - spectacular sea scapes all the way up the coast. The cliffs at Slieve League are much more dramatic than those at Moher, and with about 2% of the crowds.
To answer your question - the Northwest is much less "discovered" than either Dingle or Kerry, and believe me that's a good thing.