Hi Dean, here's what I have on William Cowan...
William Cowan married my great great aunt, Mary Stewart. Mary's sister, Elizabeth Stewart (my great great grandmother), married John Duncan.
John Duncan and William Cowan both lodged with their families at 9 Damside with the widowed Mary Stewart, nee McLeod (grandmother of William Cowan's wife Mary). Grandmother Mary Stewart died in 1876 and the Cowans and Duncans continued to live at Damside. Between the two families there were 16 children at 9 Damside before the Duncans moved into their own home about 1890. The Duncans rented their own home at 20 Duke Street. The Duncans then moved to 12 Duke Street by 1895.
William Cowan born about 1853 in Ayr
1861 Census: Age - 8. Residence - Shaw Cottage, St Quivox and Newton on Ayr
1871 Census: Age - 17. Occupation - Railway Engine Cleaner. Residence - Damside, Stewarts Land. William Cowan is lodging with Mary Stewart, grandmother of his future wife.
1872 Nov 22. Marriage to Mary Stewart, Whitletts, St Quivox Parish. Occupation - Stoker. Residence - Damside, Newton Parish. Parents James Cowan, Ploughman and Martha Cowan maiden name Hodges.
1875 Scotland Valuation Roll: Residence - Damside. Occupation - Fireman. Proprietor Mrs Anthony Stewart (Mary Stewart, widow) owns two houses side-by-side. Mary lives in one, William Cowan is the tenant of the other.
1881 Census: Age - 27. Residence - 9 Damside. Occupation - Railway Engine Driver. Co-tenants with the Duncan family
1885 Scotland Valuation Roll: Co-tenant at 9 Damside with brother-in-law John Duncan. Proprietor recorded as Anthony Stewart, Blacksmith, of 54 Beansburn, Kilmarnock
1891 Census: Age - 37. Residence - 36 Nelson Street. Occupation - Railway Engine Driver
1901 Census: Age - 48. Residence - 32 Nelson Street. Occupation - Railway Engine Driver
1916 May 12. Death. Cause of death: Chronic Bright's disease. Died at: 20 Duke Street.
I hope there is something new in this information for you.
Regards,
Andrew