There are so many McKays/McCays in the Limavady area that it is difficult to sort them out. Alexander's occupation seems to have been a labourer but in 1911 census he has become a retired land steward! Alexander McKay died 15th Nov 1915 and is buried in Dundonald Cemetery. Elizabeth, his wife, died 22nd Nov 1909 and according to an American source is buried at Lislane.
I checked GRONI today and the mother of the two Joseph McKays was McQuiston so I presume the one born in 1870 died. I also found a Matilda McKay who died 22nd March 1882 aged 3 yrs and an Annie McKay who died 2nd June 1887 aged 18.
I also confirmed that the father of Alexander who married Jane Harrison was Alexander McKay, a labourer. Alexander, the younger, was a bootmaker and he lived in Limavady.