We also use “taking coals to Newcastle” here in the Midwestern U.S. (Something about being descended from former British colonies…) But I’ve heard plenty of riffs on that idea, like, “bringing corn to Iowa.”

We would say ag tabhairt liúdar go Toraigh (carrying coalfish (pollachius virens) to Tory Island)

Or ag cuimilt saille/blonóige de thóin na muice méithe (rubbing lard on a fat pig’s arse)

Bringing even more flags to :estonia-cool: especially in the last 2 years. The flags and colours are bloody everywhere and not one has been removed.

Bringing sand to Sahara

I think “selling ice to Eskimos” is the American equivalent. Naturally, the version of this phrase: 1) involves an exonym that’s a bit offensive 2) kinda makes doing a pointless exercise sound like a good thing.