OK, here's the scoop on Megan. In the Beginning, there was MEGAN. This was a lovely Welsh pet form of Margaret. Then, the Australian writer Colleen Mccullogh wrote "The thorn Birds." I think she was single handedly responsible for suddenly convincing the world that this name was Irish, by spelling it Meghan (?). Now, everyone thinks it is an Irish name, and lots of Irishoid spellings have popped up. But, the fact remains...whether you spell it Meghan, Meaghan, Meagan, Meggyynnn etc., it was still originally a Welsh nickname for Margaret.
A lot of baby name books list Megan as being Anglo-Saxon meaning "mighty" or something like that. Don't believe them. They are all crack heads. I'm not sure where they got this idea from. (Maybe from the Latin prefix "mega?")