Jokes
There are several clichés about Ireland that tourists believe on a regular basis. It's advisable to avoid bringing up preconceptions if you want to make the greatest impression on locals. Locals are weary of hearing the same old leprechaun jokes, even if you are probably joking.
It's also a good idea to avoid making jokes about anything that happened in Irish history. Because the scars from tragedies like the Potato Famine are still raw, it's best not to insult someone with an inappropriate remark. For example, quizzing about potato would upset Irish people. Because this is a recurring gag about an Irish stereotype, but there are a few of points of order to be made.
For starters, Ireland has a lot of delicious food: try beef, salmon, oysters, butter, lamb, strawberries, and blood sausage, for example (okay, you might not be queueing up for that last one). Second, while potatoes are the local staple carbohydrate, they are also strongly associated with the Great Famine, with potato blight leading to the deaths of a million people and the departure of another million, resulting in a population reduction of over 25%. Sure, it was in the 1840s and 1850s, but it's a little delicate culturally.