According to the Annals of Richmond County and Vicinity, Vol. 2, page 153, pub. by Richmond County Historical Society (1968), Mac Sennett's parents were John Sennett and Kate Foy, daughter of Michael Foy who emigrated from Ireland.
I had always believed that he was born in Tingwick, a tiny village a few miles north of Danville (where I was born, but quite a few years later!), but I could be wrong on that.
You might try the Richmond County Historical Society at
http://www.interlinx.qc.ca/~e-dhealyKen Healy