There is just one baptism of a Margaret Smith in Galston between 1780 & 1895 and this is of Margaret Isabel Smith to just George Smith on 27/7/1783, which would mean she was 27 when married - quite possible .
There are also baptisms of John 1776, Ann & Agnes the same day 1777, Andrew 1781, Jean 1785 & Henrietta Scott 1787 , all to George Smith. Then William 1789 & Alan Freer 1791 to George Smith & Marion Freer. SP.
No marriages of a George Smith in Galston 1767-1776
1810. Margaret Smith married William Newton at Galston Ayr on 3/8/1810 . Thus would think she was likely born ~ 1790.
Thay had William Newton 1814 & John Newton 1816
Both Margaret & her husband almost certainly died < 1841, as I cannot find them in that census.