Thanks both of you for your replies, I agree largely with what you say, except that I know from studying the parish registers including my own family that families not only moved around the parish, but moved from parish to parish often, and sometimes even to different counties.
My Barbara Mackay was wife of Alexander Munro, there is no parish marriage record for them, and the first record of them is in 1791 at the baptism of their first son James Munro in Ceanabeinne, Durness, in which Barbara is described as the "wife" of Alexander.
However, by 1796 they had moved to Armadale, parish of Farr where they had another son called James, with the first having died young. In Armadale they had a total of 6 sons and one daughter between 1796 and 1812.
Barbara Mckay is found aged 75 on the 1841 census still in Armadale. Ages on the census having been rounded down to a multiple of five means that she could have been aged between 75 and 79 and thus born between 1762 and 1766. However, I have reason to believe that the age on the 1841 census is not only wrong but too high: Firstly because I have found in the Sutherland Papers a petition from my ancestor Barbara Munro (nee Mackay) dated September 1837 in which she describes herself as being in the "66th year of her age". This would surely mean that she had reached 65 years old and was approaching 66 years old, thus meaning that she was born in 1771 or 1772. However, if by saying she was in the "66th year of her age" she meant that she had actually reached 66 then she would have been born in about 1770 - 1771 - which leads me to the above mentioned parish record in Uaibeg, Durness in 1770.
Also, if I was to accept the age for her on the 1841 census that indicated that she was born between 1762 and 1766 then that would mean that she would have been aged between 46 and 50 when she gave birth to her last child in Armadale in 1812 - highly unlikely. I think the age on the petition of 1837 is much more likely to be accurate.
I am assuming that Barbara was born in the parish of Durness based on the fact that that is where she and Alexander Munro had their first child Jame Munro in 1791. Further to this I have found with genealogy that usually the man of the family has traveled from elsewhere, married and settled down somewhere other than where he was born, in most instances. But with the women they usually stayed put, married and had kids near to where they were born. I am not saying that was always the case but often was. I have information that Alexander Munro removed from his "native county" of Ross-shire in 1784.