My Brennans settled in and around Holland Township in Grey County. I have found records of their families in Sullivan, Sydenham/Owen Sound, Derby, and more, and many references to burials in the Roman Catholic Cemetery in Dornoch/Griffin's Corner.
My great great grandfather came down to Berkeley, California, but it seems many of his siblings and cousins remained in Grey.
Some of the families they married into are:
Jordan
Ferguson
Malone
McCartney
Keyes
Vasey/Vassey
Smyth
Fogarty
Coffey
Reid
Patton
Longhurst
Bradley
Peters
Flavell
McLennan
Wright