Help with Sports fan

Buy a clear shower curtain, some spray paint, ask at a local home store what kind will stick to plastic, you may need to spray some sort of primer first. It is just for the short term so don't be too overly concerned.
Make some stencils - find some celtic themed designs - preferablly easy to copy - blow them up on a printer, trace and cut out design on sticky backed paper. Spray and remove design when the paints is dry but not set.
Voila

Oh yeah, If you can sew, buy a celtic's sheet, hem it to fit the shower and it will work well as a shower curtain. It will keep the water off the floor.

The Celtics sheet is a great idea. Ikea and Target both carry clear shower curtains for cheap -- just have two layers, clear shower curtain (inside the tub) and fabric (outside the tub.) Both can hang from the same rings. You can get a cool grommeting thing for about $25. (To make the holes in the fabric for the rings.)

Hmm, haven't found sheets yet, but this has some interesting stuff:

Celtics bedding and stuff (not sheets though)

This is all I could find in a shower curtain.

http://www.buybathware.com/145007.html

I don't know much about boston, but up here you can generally find sheets and stuff in the kids section of a department store. Sears maybe.