I really don't think
Mavourneen is supposed to be a name any more than Sweetheart is. It's just it's weird. Incredibly cheesy, like earthnut said. But now I'm thinking of
Carina...
Carina is a fairly popular name and that, like mavourneen, is an endearment, I think in Italian? Which makes me wonder if Italian speakers would find using
Carina as a name weird or not. And if
Carina is weird for Italians but okay for English speakers, maybe
Mavourneen would be okay for anyone who wasn't Irish? And in the same topic, do French speakers use
Cherie as a name?
... kind of rambling, really. I probably could find out the answers to those questions if I did a bit of research, but I'm feeling supremely lazy today.