It's one of the McDonald/McDonnell tartans sewn into a Glengarry hat. I've never seen a Glengarry madeout of tartan before this one.
Ooops, I'm wrong and you're right. It is McDonnell of Glengarry, made into a Glengarry. Sort of a tactile pun.
--dbh
When given a choice, most people will choose.