I've seen that "seo dhuit-se" sugestion alright. It sounds rather like a reverse etymology to me. I've never heard it as a toast in natieve-speaker Irish and know of no evidence that it was ever common enough to have been transferred into English. The strong Dublin link also makes it unlikely.

I think its just one of life's little mysteries (As distinct from Leif's little ups an downs - an even more obscure Dublin in-joke!).

Regards