I did that one time many years ago, same route, probably same type of cargo, and very similar airplane. We didn't evac on the runway after the divert though. It was apparent when we were still flying fine, depressurized and on O2 at FL250 with no fumes or smoke after 30 minutes, that it was probably a false fire warning. This was especially apparent after we sent supernumeraries to the main deck with portable O2 who reported no heat or smoke of any kind. Even so, we deemed it best to follow through with the diversion since the FIRE MAIN DECK master warning was displayed continuously. Of course we were maligned by a few for failing to continue to planned destination, but we eventually did, after a pub visit and rest time in SNN. If they had the warning go off at the same spot we did, right at 30W, they probably wanted out of the airplane to change their undergarments, much like us.