I'm afraid that's the problem I'm having too, Duncan doesn't appear on any census, and I have no idea where or when he was born.
It's a total mystery. I know he was married on 18 November 1874 and I know he died in 1891.
I know what his parents names were from the information on those two certificates - but I can't find out anything else about him.
I know Duncan and Christina had a son, Murdoch born 08 August 1875 in Rireabhach, Lochbroom. Duncan's domicile is shown as Stornoway, Lewis. The informant of the birth was Christina's father, Kenneth.
On 03 April 1880, my grandmother, Mary Ann was born in Shore Street, Ullapool. The informant of the birth was Duncan himself.
On 17 June 1881, another son was born, Kenneth John again in Shore Street, Ullapool. The informant this time was Alexander Mckenzie, maternal uncle from Rherivoch (a different spelling of Rireabhach).
Duncan remains a mystery.