I have information on the descendents of David Barr and Agnes Airth. Dallas Manicom's website is an excellent source of Airth, Barr and McLaren lineage.
I am a direct descendent of Agnes and David via my paternal grandmother, Marion Stewart (b. Feb 9, 1896 d. Nov 4, 1986) who was married to Donald C. McLaren and they resided in Renfrew. Marion was the daughter of Peter Stewart and Annie Barr (b.Dec 15, 1874 d.May 11, 1903)- they resided in Renfrew. Annie Barr is the daughter of William Barr (b. Oct 15, 1836 d. June 25, 1910) and Sarah Dalgliesh. William was the son of David Barr and Agnes Airth.
Marion Stewart's husband, Donald McLaren (my paternal grandfather) was the son of Mary Airth (b. Oct 30, 1836 d. Mar 2, 1910) and Robert McLaren. Mary was Agnes Airth's younger sister. My grandparents were first cousins twice removed (if I did my calculations correctly).
Excerpts from information I have regarding John Barr: "Nov 16, 1851 - Nov 13, 1942. Born on Lot 8 con 2 Horton Twp. Resided on Lot 6 Con 4 Bromley Twp. Elder of Barr's Settlement Presbyterian Church and later Welsey United Church in Bromley. He was an excellent farmer. Buried in Thompsonhill Cemetary, Renfrew."
The Thompsonhill Cemetery fell into disrepair quite a few years back and a memorial stone with the names of all who were buried in the cemetary was erected. I do not have a photo of it, but will try to get one when I go back to visit my folks. I now live in British Columbia.