I suppose this advice is impossible in Ireland if you have your heart set on going to a bar since all bars by definition are Irish.
I don't drink much anymore for health reasons. When I was younger, I usually visited a bar on St. Patrick's Day for a pint or two of green beer. I'm not Irish, but hey I never missed an excuse to celebrate.
I have made the mistake of going to an Irish bar on St. Patrick's Day. For one, there is usually a cover charge just to get in to the bar. Unless I'm going to be seeing boobs, I am not paying a cover charge (not that I would visit such a place, of course). The bar will be packed and noisy. There will be long waits to get overpriced drinks. It's just simply not worth it.
The same rule would apply to a Mexican bar on Cinco de Mayo. Although, it appears this holiday is much more popular in the US than it is in Mexico.
By the way, my stock answer when someone asks me whether I'm wearing green on St. Patrick's Day is the following: "Does gangrene count?"
Nevertheless, Happy St. Patrick's Day everyone!