"Top of the morning to you" Is it formal or informal?

In the UK, this is stereotypically Irish. A non-Irish person saying it would appear very strange, unless they were clearly making a joke. (How much Irish people really say it, I don't know). Formality doesn't enter into it.

Always thought it was an Irish expression because of how people spoke it here, but what would I know. I’m Maori - native New Zealander!! Haha! We say it as a bit of a joke, like how we would imagine a leprachaun ?? would say it. Ok, I don’t need a lesson on leprachauns!! ?? But here in Kiwiland, we consider the use of the expression a cheery way to greet someone at the beginning of the day. (Not sure if moderators will accept my post, but that’s all good!)

This phrase appears to come from the 1600s in England. There does seem to be some debate as to the possibility of it coming from the Irish language, however people from Gaeltacht areas(irish speaking villages) have told me that this is not true. It appears to have been associated with Ireland in America, and thus Irish americans started using it.