Mcphails IN North Uist Outer Hebrides Scotland Late 1700 S

I am descended from Mary McPhail and her husband, John MacDonald, who were both born about 1790 in North Uist, Scotland. They had a son, my ancestor, Lochlin MacDonald, born 1819 or 1829, in Tighary, N. Uist. I would like to be able to trace Mary's lineage further back...thanks in advance. Helen T.