There is also a John McMillan buried with Dougald and Ann in Erin Union Cemetery, 1868-1942.
The cemetery lists his year of birth as 1834, the 1901 census has Dec. 10, 1838.  

From the "Biographical sketches" Historical Atlas of the County of Wellington, 
Ontario, compiled, drawn and published from personal examinations and surveys.
Toronto: Historical Atlas Publishing Co, 1906, reprint 1972.

Dugald, was b. in Erin Tp., in 1839, and has always lived there.  He succeeded to the
homestead, which he retained until the early ninties, when he sold it, and purchased 
lot 23, con. 9, which he has greatly improved, and where he still resides.  He is a 
successful man, and good farmer, having the respect of the community at large.  He
m. Ann, dau. of Donald McLean.  Issue: Sarah, Mary A., Daniel, Mrs. Albert Rooney, 
Toronto; John, Alexander, and Christina.

Erin Advocate, Apr. 24, 1912 McMILLAN - The late Dougal McMillan who died on the 11th inst. was a life-long resident of this vicinity.  He was the second son of the late Alexander McMillan and was born at Belfountain in 1834.  The family afterwards removed to the 9th line Erin, where he resided until his death.  His wife, who survivies him, was Ann McLean and a family of three sons and four daughters were born to them: Alex, of Eramosa; John and Daniel, at home; Chrissie, Mrs. Burnsides, of Ballinafad; Mary Ann, who died some two years ago; Maggie, Mrs. A Rooney, Toronto; and Sara, Mrs. W. Wheeler, tenth line Erin.  The deceased was a house carpenter which occupation he followed until about ten years ago.  He was a Reformer in politics and was a member of Burn's Presbyterian church at Erin.  His death was caused by cancer and towards the end of his life he was a great sufferer.  The funeral took place to Erin cemetery, on the 12th inst and was largely attended.  The pallbearers were, Messers A. McGill, J. Wilson, E. Stowger, R. J. Lang, and Donald and Duncan McLean.  Rev. R. S. Scott, of Hillsburg, conducted the services at the house and at the grave.  The bereaved family have the sympathy of all.