If Samuel and Mary Ann had any children born in Scotland in 1855 or from 1860 onwards, the date and place of their marriage should be recorded on the children’s birth certificates. If that was in Ireland, then obviously that will help you narrow the search for their roots.
Check all the Scottish censuses for the family in case it gives an exact place of birth in Ireland, rather than just Ireland. Sometimes it does.
There’s a death for Mary Ann Lackie formerly Chambers, in Dalmellington 1870, aged 53. GROS 586/00 0139. Might be worth checking to see if that’s your Mary Ann. It should tell you whether her husband was still alive at that time, and so narrow the search for his death. You should also get her parents names which may help you.