It depends on when @sparky42
. Patrick Buckland is good on this. Basically, in the halcyon days of the1920s and early 1930s, when Britain has economic troubles and no serious military threats, NI is seen as an unnecessary expense and even the Tory Right like Joynson -Hicks would have been quite happy to divest themselves. 1936 on, this becomes more muted with a resurgent Germany and Russia. 1940s, Derry is too important as an Atlantic port and manufacturing in NI is needed to help equip the war machine, unification thus only being offered on "you would have to join the war" terms.
1950s and 1960s, NI is too important to NATO because of Bishopscourt, Eglinton, Aldergrove and Gilnahirk and is quietescent if expensive. Early 1970s, desirable, but too sensitive an issue to raise. Late 1970s and early 1980s, "unification if you agree to joining NATO and rejoining the Commonwealth"(wouldn't have worked but demonstrates British interests are strategic and not economic). Mid to late 80s Eglinton no longer needed for MR, Aldergrove no longer necessary for fighter and bomber cover, Bishopscourt made obsolescent by A EW, Gilnahirk by improved monitoring technology and unification without preconditions is desirable again. Not readily deliverable but FCO and Whitehall generally in favour.