The terms "a Leeroy", "doing a Leeroy" and "to be a bit of a Leeroy" etc, are easily confused with Leeroy Jenkins (who became infamous from World Of Warcraft for leaping headfirst into a situation regardless of strategy), hense the use of the full term "Leeroy Le Gallais". Usually the short version "a Leeroy" is used when context will suffice to explain.
The insult was born after public outcry in Guernsey when a Guernseyman hit national headlines when he was taken to court for molesting a horse (locals couldn't believe that some freak would actually do this, Guerns aren't all like him!)
"What are you, some kind of Leeroy?"
"He looks futu! I reckon he spent all last night Leeroy Le Gallaising"
"That mare looks like its been Leeroy Le Gallaised"
"You've got to remember - they're a bunch of Leeroy Le Gallaises!"