About T-Shirt Design
Slainte Irish Shirt works because it sounds more like something a real person would say than a random slogan. It fits really well with St. Patrick's Day, but you do not have to be obsessed with that stuff to enjoy the joke.
You could wear it for St. Patrick's Day, bring it out for parades, or just keep it in the normal shirt rotation. It works well as a novelty t shirt because the joke is clear and the design has plenty of personality. This is the kind of idea that works well on novelty shirts because you can understand it right away. It feels casual enough for normal days but still has enough personality to get noticed.