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I have found your question (six years after you posted it -though I have seen it before) - But if you do check this, even now, here's what I know. Many, many (MANY) years ago, when I was a student at Boston College, I had the same question. And, since Boston College has one of the most extensive libraries on Irish History in the country I decided to go check it out. The librarian there (she was very old and had been there forever)found a research book and this is what we learned: Glennon is derived from MagLeannain - translated as "son of the one who wears the cloak". Further research led me to learn that the "cloak" was a leather cloak, tanned and treated to be water resistant. It was a cloak worn by "farmers". It was treated so that farmers could still work their fields, even in the rain, and not "get wet". So I guess it means "son of a farmer - or farmer". My aunt on the Mahan side of my family claimed that we Irish are all decendants of Brian Barou, and therefore are all decendants of kings! :) Though being a "descendant of royalty" may sound good, I sort of like the idea of descending from farmers! You'd think we all would have a green thumb - however, somone needs to tell my vegetable garden that - as they apparently do not seem to know that I "have the heritage!" :)