NameMary McKay 
BirthAugust 1818, Scotland
Death29 May 1910, Nova Scotia, Canada
Spouses
Birth1811, Scotland
DeathFebruary 1861, Nova Scotia, Canada
Notes for Mary McKay
Andrew Roberstson gives her lifespan as 1818-1910.
Canadian Census. Mary McLellan is listed in the 1881 Canadian Census as a widow, her husband has died. She is age 61 and was born in Scotland. I am certain it is Stewart's grandmother, despite the spelling anomoly, because Duncan is present in the household and all the infomation about Duncan fits with what I know. In addition to four of her children living with her (Bella 32, Duncan 28, Alex 26 and John 23), a 12-year-old Mary McKay lives with her -- apparent confirmation of the fact that Mary's maiden name was McKay. The young Mary McKay is probably a niece or great-niece.
I have not found Mary in 1891, 1901 or 1911. She may be present in the Census and just hard to identify without Duncan present in her household or she may have died.
Notes for Donald (Spouse 1)
Andrew Roberstson says Donald MacLennan (1811-1861) was of River Denys. He says he was killed "in the woods".
Candian Census. The earliest Census I have seen for Canada is 1881 and Donald had died before then. The 1891 Census asked people where their parents had been born and Duncan MacLennan said both his parents had been born in Scotland.
There is one Internet Family Tree with some of Stewart's ancestors in it. It is the tree of George MacLennan -- and like most such trees, not many sources are listed. It comes forward in time on the branch of interest to me no further than Donald MacLennan who married second Mary McKay, but it lists no children for them. So for what it's worth, here is what it says about Donald and Mary and their ancestors.
I have picked up all relevant dates and places from this tree, but they are only as accurate as George's research.
Hugh McLennan b.1760 Scotland, resided Locharron/Lochalsh married Isabella MacKay bef. 1770 in Scotland.
Their children include Alexander Ban McLennan (aka Alister Ban) b. bef. 1800 in Lochcarron and immigrated to Pictou, Nova Scotia about 1824 onboard the "Tamerline". He married Barbara McLennan b. abt. 1791 in Rosshire. She was the daughter of Neil McLennan and his wife Rebecca. (These latter two names are at variance with Andrew Robertson's information.)
Among their children is Donald McLennan born abt. 1811 in Lochcarron. He died Feb 1861 in Grand River Falls, Nova Scotia, killed by a fallen tree. He married first Catherine MacKay who had been born in Nova Scotia and married second Mary MacKay (no apparent relation to Catherine) born Aug 1818 in Plockton, Scotland. Mary was the daughter of John Ruadh MacKay and Ann Kennedy. Mary died 29 May 1910 in Glace Bay or Grand River, Nova Scotia.