Leo Walsh

From Memory Alpha, the free Star Trek reference.

Leo Francis Walsh is an alias used by Harcourt Fenton Mudd in 2266. Mudd was then engaged in the business of providing wives for settlers on Ophiucus III. According to Mudd, Leo Walsh was a real person; the man originally slated to pilot his ship, who "passed away, suddenly". Mudd claimed he took Walsh's name "in memoriam, as it were". (TOS: "Mudd's Women")

It is at least as likely that Mudd's extensive criminal record made the use of a pseudonym necessary.