I don't know about the story Paty305 (the BS one), because I think it IS BS. I have been associated with the club since the late sixty's (and even though I was a child) and I think the filthy micks was probably on the mark.
In the 'ol "green shed" days there were a number of wild and woolly Sunday nights there after the game; well into the night and in the pitch black at times. I think my first memory of the word was in the early to mid seventies and remember players like Tony Shaw and Budda Handy using it. Actually whoever started it, Shawey loved it, almost relished in being called it.
They weren't ruthless players, "win at all costs"; they simply hated losing and they would stick up for their mates; If you played for Brothers you played as one, and that was the strength, the belief.
Long story short, I think it was other teams calling them filthy micks. It was a different world in those days.