As described in historical dictionaries

Sorry, we don't have any historical texts which describe the McClennan surname. That's probably because it was very rare in Great Britain more than a century ago.

Surname type: From name of parent

Origin: Celtic

Region of origin: British Isles

Country of origin: Ireland

Religion of origin: Christian

Language of origin: Gaelic

BritishSurnames.uk is a Good Stuff website.