Used about a person from Ireland, Mick is a rude, hurtful word. Do not use it. It comes from Michael, a name that has been very common in Ireland for hundreds of years, and people turned that familiar name into a label for a whole country. A label like that flattens someone into a joke or a target. You may still meet it in old films, songs, or angry speech, which is why a learner’s book still has to warn you. Though actually, the picture is a little messier: Mick is also a normal short name for a man whose name is Michael, the way Mike is. Friends may call Michael “Mick” with no unkind meaning. The line you must not cross is using Mick to mean “an Irish person.” If you mean someone is from Ireland, say they are Irish.