He advocated for a soilution that would have handed the IRA victory By suggesting that the UK imposes a United Ireland on the North, against their wishes. That's why he voted against the AIA, because it left the decision up to NI voters. He rather stupidly described his reasons for voting against it:
We believe that the agreement strengthens rather than weakens the border between the six and the 26 counties, and those of us who wish to see a United Ireland oppose the agreement for that reason."
He didn't advocate "dialogue" at all, but an imposed United Ireland that would have been an IRA victory.
He believed that the AIA wouldn't do anything to bring peace, and he was right. He voted in favour of the Good Friday Agreement, and that didn't hand the IRA a victory. At some point in the future, probably actually in the nearer future than previously thought due to Brexit, there is a fairly good chance that the reunification of Ireland will happen. Does that mean the IRA will have been victorious?
In principle I would like to see Ireland returned to 32 counties in my lifetime, although not at the cost of peace. Essentially I guess I am pro unification. In that sense I am on the same side as the IRA - does that make me one of them?