75 at death in 1887, if I have the right person,

I am fairly sure this is him:
Name    WILLIAM MACKEY
Year of Death    1887
Group Registration ID    N/R
SR District/Reg Area    Cootehill
Deceased Age at Death    75
Returns Year    1887
Returns Quarter    2
Returns Volume No    3
Returns Page No    113
At the marriage of his son, Robert, he is described as deceased.

100 at death in 1906 but only 90 in 1901 Census.

Name    MARY MACKEY
Year of Death    1907
Group Registration ID    4894334
SR District/Reg Area    Cootehill
Deceased Age at Death    100
Image
Mary Mackey, of Dunnyluck, died in 1906 aged 100, a "soldier's widow, Robert Mackey, also of Dunnyluck, PAD.

Gravestone says she died 20th Dec 1906, aged 102!

55 in 1901

Source: gravestone in Drum Presbyterian church, which says he was 80 and from Drum, NOT Dunnyluck!!