I note from William’s 1865 marriage that he was a widower, so he obviously had an earlier wife who had died. Statutory marriage registration started in 1845 in Ireland. I searched 1845 – 1864 but did not find one for him, so either he married before that or outside Ireland.
According to some trees on Ancestry, Rev Acheson Findlay died 2.4.1888 in Fort George, St Andrew, Jamaica. Depending on what his death certificate contains you might get some extra information from that. I am guessing he was a Presbyterian Minister. Have you searched Fasti for any records of him? It wasn’t possible for Presbyterians to get a theology degree in Ireland until the late 1840s, so most went to one of the 4 Scottish universities to get their degree (Glasgow, Edinburgh, St Andrews or Aberdeen). They all kept good records and often have a little personal information about student’s origins. Could be worth exploring. William Finlay senior was reportedly a clerk, according to the 1865 marriage certificate.
If you think the Rev Acheson may have been a Minister in Ireland, then the Presbyterian Historical Society in Belfast should have some information on him:
https://www.presbyterianhistoryireland.com