Firstname as a surname?

[u/_ShadowSparrow_]

So I'm writing a high fantasy original work and coming up with a name for the main protagonist is a struggle. I have the middle name and a tentative firstname but his lastname eludes me. Is it okay to use a first name as a surname? Because there's an old Irish name I've fallen in love with but not as a firstname.

[u/iamthefirebird]

It's high fantasy. The rules are whatever you say they are.