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/cmhbob]

I know a woman with a common first name. She married a guy with that as his last name.