About T-Shirt Design
If you like shirts that actually have some personality, You Need Some Irish In You T Shirt is a good example. Anyone who is into St. Patrick's Day will probably understand the idea right away.
You could wear it for St. Patrick's Day, bring it out for parties, or just keep it in the normal shirt rotation. For a funny t shirt, it keeps things pretty straightforward and lets the saying carry the humor. It has the relaxed feel people usually want from novelty shirts without getting too complicated. It is not trying to be deep. It is just a funny shirt with a good line on it.