Scottish indexes have an entry in their paternity index;
Pursuer : Mary Ann Dalrymple, domestic servant, Muirbank, Ecclefechan, Dumfriesshire,
Defender: William Beattie, Porter, ( Caledonian Railway Company), Buchanan Street, Glasgow
The decree was in 1892 ( thus after 1891 census when William Dalrymple from Forfarian's post is showing as second cousin).
Suspect, Kay, that Muirbank has been wrongly transcribed as Meinbank in the 1891 census you looked at.
I would also point out that he was born before, not after, the Johnston/ Irving marriage.
William
( Was typing while you posted, Neale).