I've just had a closer look at the death certificate and in fact he is put down as being married to Mary Bridges - not a Mary Grierson. Which is a puzzle.
Indeed.
I wonder if Bridges might be Mary Grierson's mother' maiden name? There are 4 baptisms of children to John G and Agnes Bridges in Tinwald between 1810 and 1818. And a grandmother's maiden name instead of a mother's is probably the most common error I've seen in death certificates.
In 1841 Hugh is 22, so born 1818/1819
In 1851 he is 34, so born 1816/1817
In 1861 he is 46, so born 1814/1815
In 1871 he is 55, so born 1815/1816
On the death certificate he is 64 so born 1821 (or, just possibly, 1820 but the death date is so close to the end of the year that it's unlikely)
On the stone Hugh is 73, so born 1812 (or, just possibly, 1811)
1811/1821 is quite a span of years! Especially between the certificate and the stone.