Source: marriage entry in Coleraine Local Record Office - married in 2nd Dunboe, resident of Letterloan, full age spinster, daughter of John McKeown. Anna Bella McKeown witnessed marriage (along with Henry Leighton). If this were my great grandmother, she would only be 17 at the time of the wedding.
N.B.: Placing Jane as the daughter of this John is speculative - Jane could also be the daughter of the younger John McKeown (son of John), except that the younger John's wife, Martha Dooey, was supposedly born in 1848, which makes her too young to be the mother of someone marrying (aged circa 20?) in 1883.
Not in either census.