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. They were cousins once-removed. Her parents were David Magill and Anne Wilson.
Do you or your mother have any data on George & Mary Craig Kenny's son Howard 1923-200? He was my father's cousin. He married Patricia Glynn in 1968 but I'm not sure when he died. And can she go back any further than Thomas Kenny and Eliza Douglass in the mid 1750's?