I think on a previous thread where this came up I did discount IRA bombers who managed to blow themselves up while making their bombs or transporting them. Had your 'volunteer' and the organisation he represented really wanted to protect civilians they wouldn't have been building the bomb in the first place.
I do not believe that the UK armed forces ever deliberately underestimate any opponent. Your point is what exactly? Fought to a standstill in a situation where at no time there never was ever going to be a military solution or 'victory', the PIRA negotiated a cease-fire in 1994, this briefly broken and resotored, 1998 saw the GFA, 2005 saw the P O'Neill statement renouncing the armed struggle and decommissioning of PIRA weapons. As previously stated the UK and Irish Governments coulld easily have continued to maintain the stalemate - The PIRA could not, especially after 9/11.