Well.
The obvious source for a death in 1859 is the statutory registers of deaths at
www.scotlandspeople.gov.uk.
Unfortunately there is no death of either an Ann* M*Lean or an Ann* Math*son in Contin in 1859. There isn't even anyone vaguely resembling this either.
There is a marriage of Donald McLean to Ann Mathieson in Killearnan in 1846. This might be the Donald and Ann McLean who are in the 1851 census in Urquhart and Logie Wester.
There is a birth of a Harriet McLean to Donald McLean and Ann Matheson in Contin in 1873, and there is a marriage of Donald McLean, son of Hugh McLean and Jessie Matheson, to Ann Matheson, daughter of Alexander Matheson and Harriet Simpson, in Contin on 5 July 1872. In 1881 they were in Jamestown, Contin: Donald McLean, 36, shoemaker, born Contin; Ann McLean wife, 32, born Fodderty; Alice Gillespie, stepdaughter, 11, born Glasgow; Harriet McLean, daughter, 7, born Contin; Hugh McLean, son, 6, born Contin; Alexander McLean, son, 2, born Contin; Ann McLean, daughter, 5 months, born Contin; Jessie Cameron, servant, 16, born Fodderty; Alexander Fraser, boarder, 63, born Kiltarlity. Ann* M*Lean, other surname Mat*son, died in Contin in 1883, aged 34.
So there were two couples with the same names, married 26 years apart, but the second couple isn't the one you want.
Ann McLean, other surname Matheson, died in Urquhart and Logie Wester in 1886, aged 92. Ann Matheson, mother-in-law, aged 86, is in the household of Alexander Fraser and Mary Matheson in Urquhart in 1881, so this is Ann Matheson, widow of Donald McLean, not the other way round.
Another Ann Matheson or McLean died in Urray in 1899 aged 63, but I can't see her easily in the 1881 census. In any case this one is much too young to have married in 1846. (Not the widowed Ann McLean in Urray in 1881; she was Ann Bartlet.)