Do you think this family is related to the one in the rest of the thread? Or should it be in a separate thread of its own?
In the 1871 census there are two Ann McDonalds born 1785-1805 in Harris.
One aged 76 with her son Neil, 32, all born in Harris.
One at Little Urgha, aged 74, with son Murdo, 50, unmarried.
In 1851 at Urgha are Ann Macdonald, widow, 60, and her daughter Mary, 24.
At Kyles Scalpay are Neil Macdonald, 20, with his mother Ann, 60 and three sisters.
At Geocrab are Farquhar, 45, wife Ann, 40 and six children, none of whom match yours.
In 1841, at Tarbert, are Farquhar, 40; Murdo, 20; John, 18, Catherine, 40; and Mary, 15.
At Givcrabt (which I take to be a mistranscription of Geocrab) are Farquhar, 35; Ann, 30; Christian, 10; Alexander, 8; Ann, 5; Donald, 2.
At Kyles Scalpay are Murdoch, 60; Ann, 55; and seven children including Neil, 7.
At Obb (Leverburgh) are Rod(eric)k, 30; John, 60; Malcolm, 2; Rachel, 25; Ann, 40.
So either your Ann was misrecorded as Catherine in 1841, or she was visiting at Obb, or she was away from home, or she was simply omitted from the census.
Urgha is on the north side of Loch Tarbert, about two miles from Tarbert, so it's not beyond the bounds of belief that it could be enumerated as Tarbert.
https://www.geograph.org.uk/photo/201579 - Little Urgha is Urgha Beag, of course,
beag being Gaelic for little.
https://www.geograph.org.uk/photo/501509So there don't seem to be many records of Farquhar MacDonald, and indeed if Bill Lawson hasn't found any, there are probably none to be found.
However Ann is recorded as a widow in 1851, suggesting that 1854 in his book may be short for 'before 1854', 1855 being the start of statutory registration of births, marriages and deaths, and that he actually died between 1841 and 1851.