As promised here is what I have concerning your line of Dennenys.Most of what I have below is from the parish register of St. Columban's Church in Cornwall, I also have a number of Cornwall city birth and death records and will look through those later today to see what else there is.
From what I've been able to find the Dennenys came from Co. Monaghan Ireland and emigrated to Canada - there seem to have been four brothers and possible a fifth; they were Edward (my gr gr grandfather), James (your gr gr grandfather), John, and Patrick.From the 1851 and 1861 census returns they were all living with or near each other which makes their relation almost certain.The fifth was Frank Denneny of Milles Roches - descendants of this line have the sames stories and also are acquainted with the other Denneny descendants.
Your gr gr grandfather James was married to Catherine ? and they had at least three children: Catherine born 8/21/1843, baptised 9/17/1843 (sponsors John Murphy and Ann Denneny) Owen born 4/28/1837, baptised 5/4/1837 and Patrick your gr grandfather (no date of birth given)
Patrick married Margaret Lally, daughter of Francis Lally and Catherine Snetsinger, on 4/25/1881 in St. Columbans and they had at least three children; Clarence Murray born 6/15/1886, baptised 6/27/1886 (sponsors John Denneny and Ellen Lally) Frederick M.born 2/3/1882, baptised 2/12/1882 (sponsors John Lally and Mary Murphy) and Mary Edith born 10/7/1883 baptised 10/21/1883.
For both Frederick and Mary Edith there were notations in the margin in parish register entry; Frederick "married to Annette Turnele (perhaps its Quesnel but not sure) May 28, 1923, Sault St. Marie" Mary Edith "Married to William Maloney, June 6, 1914, in St. Joseph's Church, witnesses were Frederick Denneny and Basil O'Meara"
Also, in the parish register for James Denneny (your gr gr grandfather) there is the following burial note; "James Denneny, buried 6/4/1850buried in this cemetary who was killed by an accident at the Locks above this place last Sunday being 47 years of age"
Let me know if all this makes sense - you are also related to Cy Denneny, member of the hockey hall of fame, top scorer in the NHL in the 1910s and 20s and Boston Bruin coach who led them to their first Stanley Cup.