Not sure which years you're relatives lived. My Grandfather was Robert Kenny 1882-1955. Robert's parents were Douglas Kenny 1837-1926 and Hannah Montgomery 1845-1926. They had 10 children including Mathew Montgomery 1885-1970 and my grandfather but there was no brother named Thomas.
My greatgreatgrandfather was Robert Douglas Kenny 1797-1878. He was married to Martha Dale 1796-1856 and they had 8 children including Thomas born 1832 and Douglas 1837-1926 (noted above). So I believe your Grandfather's Uncle was Thomas, not his brother. The Magill connection is as follows:
Robert Kenny 1833-1922 (my greatgrandfather's brother) was married to Sarah Jane Beattie b. 11 March 1835 d. 8 Jun 1908 and they had 10 children as well. Sarah Jane's parents were Archibald Beattie 1795-1879 and Agnes (Nancy) Magill 1796-1889. Also George Kenny (the youngest of Robert & Sarah Jane's 10) married Mary Craig Magill 1873-1958. Her parents were David Magill and Anne Wilson.
Do you or your mother have any data on George & Mary Craig Kenny's son Howard 1877-1950? He was my father's cousin. Did they have any children? And can she go back any further than Thomas Kenny and Eliza Douglass in the mid 1750's?