Whether there is support or not is not the point, I'm sure there are many in this country who would not care a jot if we just gave the Falklands and Gibraltar to their respective neighbouring countries. As for Ireland well Ireland has no right to determine the future of Northern Ireland, whether that is by means of terrorism or not, it is completely irrelevant.
The point in all of this is we should be asking if Northern Ireland wants to remain a part of us and they do overwhelmingly which suggests we should defend their right to choose. The IRA sought to take away the right of those people to make a self determination by means of violence, we went to war with Argentina over the exact same thing and we would do so with Spain too over Gibraltar if it came to it.
Those who support a United Ireland or the IRA or anyone of that faction is directly opposing the rights of your fellow countrymen, rights people have died to protect. Corbyns failure to outright condemn the IRA shows that he is quite simply unwilling to defend the rights of the people he would be charged with protecting.
He has only shown that if this country was attacked he would not retaliate albeit because he is not willing to use violence to protect us from threats.