I don't believe Ronin is, strictly speaking, the "Japanese version of" Ronan. I believe (and any Japanese scholars around here please correct me if I'm wrong *Murasaki*) ronin is a noun which means "masterless samurai" and I don't think it's used as a name in Japan, because being ronin was considered a disgrace in feudal Japan. Also, again unless I'm mistaken, it's prn ro-NEEN, very different from RO-nan.