John McArthur Mary MCDONALD
Children: Ann (bap) 9 Dec 1850, Alexander (bap) 19 Jun 1855, Mary
Not sure if the above are yours Ann doesn't appear in the 51 census
But these definitely fit
John MCARTHUR b abt 1818, Oban, Argyllshire, Scotland
Mary MCDONALD 14 Sep 1846 (marriage)
Children
Flora (bir) abt 1846, Donald (bir) abt 1848, Catherine (bir) abt 1849
Census 1851
1851 Tapull, Kilfinichen & Kilvicheon
John MacArthur 33 Farm labr b Oban
Mary MacArthur 32 wife b Ross of Mull
Flora MacArthur 5 dau b Ardmeanach
Donald MacArthur 3 son b Ardmeanach
Catherine MacArthur 2 dau Ardmeanach
Angus McDonald 62 Un shoemaker b Ardmeanach
It's definitely worth contacting these people
http://www.mullgenealogy.co.uk/