Hi Alan
Welcome to Rootschat.
I believe I have found family on 1901 Census but no Donald shown - however, you don't state when your grandfather was born so this was possibly after 1901.
1901 - Minard Castle Stables, Kilmichael Glassary, Argyllshire
Donald McNab Head 36 Coachman b. Glassary, Argyllshire
Catherine McNab Wife 34 b. Kirkhill, Invernesshire
Alexander McNab Son 5 b. Glassary, Argyllshire
John M. McNab Son 3 ditto
Catherine F. McNab Dau 2 ditto
Roll CSSCT1901_178
Looked for marriage and it looks like they married 1891 Kirkhill, Inverness - Donald McNab and Catherine McKaig (??) - think this is mistake and should read McKay.
Searched the IGI on familysearch.org and found:
Donald McNab b.9/12/1864 Glassary, Argyllshire son of John McNab and Helen Campbell.
Catherine McKay b.12/5/1864 Kirkhill, Invernesshire, dau. of Kenneth McKay and Catherine Fraser.
Then checked 1871 Census:
1871 - No.9 Shirdrim, Glassary, Argyllshire
John McNab jnr. Head 40 Ploughman b. Glassary
Helen McNab Wife 45 b. Greenock, Renfrewshire
John McNab Son 16 b. Glassary
Mary McNab Dau. 13 ditto
Duncan McNab Son 9 ditto
Donald McNab Son 6 ditto
Isabella McNab Dau. 3 ditto
Roll CSSCT1871_90
1871 - Drumchardney, Kirkhill, Invernesshire
Kenneth MacKay 48 Labourer b. Margringh, Ross shire
Catherine MacKay 44 b. Kiltarlity, Invernesshire
Alexr. MacKay 13 b. Kirkhill, Invernesshire
Ann MacKay 10 ditto
Mary MacKay 8 ditto
Catherine MacKay 6 ditto
Christina MacKay 3 ditto
Roll CSSCT1871_18
The 1864 birth shows Catherine fibbed about her age on 1901 Census - this is the only Catherine McKay born in time period in Kirkhill and confirmed on Scotlands People.
Only had a few credits left on Scotlands People so have only viewed indexes there. You can now view some more certificates yourself now and hopefully will then be able to trace the family back using the IGI on familysearch.org.
Good Luck.
Annette