One website I looked at went as far to claim St. Mirren was the patron Saint of football! Sadly I think someones on the wind up there!
It is actually much, much more simple. Mirren (before becoming a Saint), was an Irish missionary during the 9th century. He came to Scotland and edned up being based in Paisley, possibly founding the first church there. It is bhelieved he later died in Paisley.
A few centuries later, some monks dedicated a parish (which I assume ended up being Paisley Abbey) to the Saint Mirin (an acceptable variant in spelling). It is thought he was the first person to preach in the Paisley area. And tha's it, basically. If you want to know more, go to http://ourworld.compuserve.com/homepages/Diocese_of_Paisley/page4.html and you'll find out all you want.
Good to see that St. Mirren has a proper day. Mark 15th of September in your diaries :-)