Records for this period are patchy at best.
Quite promising, though, is the marriage in 1854 in Newry Parish (Presbyterian) of an Isaac McCrum, son of Isaac, to a Mary. The brother of your Alexander?
Then there's Agnes, daughter of Isaac, who married George Murphy in 1854 at Downshire Road Presbyterian Church.
There's also a Joseph, son of Isaac, who married Susan Boyd in 1851, though this time the surname is given as McCrumb.
In the 1864 Griffith Valuation, there are two Isaac McCrums in Newry Parish, one in Desert and one in Finnard.
In the 1911 census, there's an Isaac in Castle Enigan, Newry Parish. He's a young man, not of the generation you're interested in, but he may be a descendant. (There's only one other Isaac in Ireland at this point, in Antrim.)
In 1903, a 23 year-old Isaac, carpenter, sailed to New York. He came from Newry.
There's a death in 1881 of an Elizabeth McCrum, but it's in Banbridge not Newry.