About T-Shirt Design
You do not have to think too hard about Drink Like An Irishman Today T-Shirt. The joke is right there and it gets the point across fast. It fits really well with St. Patrick's Day, but you do not have to be obsessed with that stuff to enjoy the joke.
It works for St. Patrick's Day and parades, but it is also the kind of shirt someone could wear whenever they feel like it. As a novelty t shirt, it has a simple idea that is easy to notice without feeling overdone.
It is not trying to be deep. It is just a funny shirt with a good line on it.