I visited County Mayo [Grandfather born near Bangor Erris - Grandmother born in Ballina]with my brother in August 2001. We stayed on the shore of an arm off Braod Haven Bay [east side] and at the Welcome Inn in Castlebar.
If I were going back now the first thing I would do is check the website "Mayo On The Move" [Google will give the link] for the latest schedule of events and concerts.
   As to places to visit [in addition to Knock, Croak Patrich, Aran Islands, Belmullet and Westport] I would recommend Ballintober Abbey [on Route N84 about 10-12 miles south of Castlebar] and the boat cruise of the Lough Corrib islands leaving Oughterard on the west side.   
If your interests include Hill Walking and/or Diving, check with Neil McEleney at his Bed and Breakfast in Pullathomas
Telephone 353 97 87800 or by EMail cunnafarraige@tinet.ie or
neilmceleney@tinet.ie   Tell him Ed and Jake Shevlin recommended him.
    Bon Voyage and let us know about your trip when you return. Ed