I am researching the Stevens family and closing in on the relationship you are seeking info about. I had long assumed these ancestors had come from Ireland in the 1800s, but it appears they were in the USA before Canada. It was one of the daughters of Stiles Stevens, a loyalist from Boston, to whom W. T. O'Reilly married.
From: History of Toronto and County of York Ontario. Volume 2, Toronto, C. Blackett Robinson, 1885, page 125.
"William T. O'Reilly, MD, was born at Niagara Falls in 1834, being the eldest in a family of two sons and one daughter. His father was William O'Reilly, the youngest of a family of six sons and five daughters, and was born in the sam place and the same house as his son. In the year 1800, he married a daughter of Stiles Stevens, a U.W. Loyalist, who came to Canada from Boston; by her he had two sons and one daughter, viz.: William T., Hellen and Hamilton; he died in Oakville in 1846. The grandfather of our subject, John O'Reilly, Came from Baltrasna, in the County of Cavan, Ireland, in 1745 to Philadelphia, where he became the president of the University of Pennsylvania. In 1786 he came to Canada and settled at Niagara Falls, where as a U.E. Loyalist he drew land for himself and sons, and where he died in 1815. He and five sons served during the war of 1812, and fought at Queenston Heights and Lundy's Lane. Dr. O'Reilly attended Upper Canada College in 1847-48-49, and graduated in medicine in 1856. He then practised in St. Mary's for a short time, but returned to Toronto in 1859, and now occupies the position of Inspector of Prisons and Public Charities for the Province of Ontario."