I fear it's because Tìr Chonaill is cut from the same cloth as the No Surrender Billy Boys, and is unwilling to move forward. Some perceived slight by a squaddie back in the mists of whenever has festered away and poisoned his view of the British as a whole. It's a form of racism - substitute Paki for Brit and you get the true picture of where he's coming from. It's not you personally, Keith; he'd be touchy/picky/volatile with anyone on your side of the water.
Sadly, folk like Tìr Chonaill are a part of the problem and belong to the past not the future. Until they can find 'closure', or at the very least open their eyes the way Ervine did - or even fade gently raving into the old folks' homes - the peace process will remain fragile. The bigots of the DUP need little enough excuse to screw things up, and the rantings of people like Tìr Chonaill are actually manna from heaven to Paisley's goons. You can hear the mantra, "See, we told you so - those taigs'll never accept change. No reason to move on..."