When the McGregor Clan was outlawed in Scotland and the McGregor name could not be used, they called themselves ... 'The Sons of the Glen' ... McGlenn's ... some of the McGlenn's emigrated to Ireland mainly the Donegal area and settled down. The only scribes/notaries at that time were the Priests of the parishes. Some could not get their tongues round McGlenn, so they were assimilated into the McGlinn/McGlynn (whether North or South of Ireland) factions/Clans.
My Grandfather emigrated back to Scotland during the troubles of the 1920's and so we are back where we started ... in his words we are a sect of the McGregor Clan.
Unfortunately my Grandfather Frank (Francis McGlinn) died long before I was of age to question him ... but it sounds as if it could be right???