All right, I have to admit I find this St. Patrick T-shirt amusing. Highly suitable for your March 17 and Celtic festival needs.
But heck, nobody is likely to hassle you about it, if you go out wearing this ferocious and beautiful picture of St. Michael. The St. Michael prayer is on the back, which will come in handy if anybody behind you has demon troubles.
(Actually, that would be pretty amusing for a horror gaming campaign set in the modern day. Team members could wear T-shirts full of printed cheat sheets. I guess you’d want to print the prayers upside-down on the front of one, to be useful to the person wearing it.)
Here’s a Rosary T-shirt. Note the characteristic Hispanic/Mediterranean wearing of the Rosary around the neck, which has lately become something of a pop fashion statement with non-Catholics. This is a nice shirt,
but it’d probably be nice to have a non-neck version for other cultures. 
Also notice the huge honkin’ crucifix, pointing out that the Rosary is all about the Christology.
I like the Annunciation illustration on this one.
And they say nobody practices mortification anymore, when people wear this out in public…. I like the mirror letters and the sentiment, but it would take a brave and troubled person to do it justice, much less conceive of it. Impressive.
I really like that St. Michael shirt…
And the inclination to do a modern horror/adventure game in a Catholic vein is *strong*. The whole ‘SWAT Team vs. Demons’ thing is well ensconced within my wheelhouse.
Hehe, my husband is working on a horror comic/campaign setting… I keep inserting Catholic reasoning and such where it fits, I’m slowly getting him hooked. ;^p