Not much is known about McGillicuddy and that's not my fault. But he's a little older than Mick and Ketch and Ketch had a brief moment of screen time with him before Ketch had to leave and McGillicuddy was killed by vampires before he could return. Happy?
In my fanfic, McGillicuddy is married to Watt. And canonly Watt and Ketch knew each other as kids and we're friends growing up. So, technically, McGillicuddy would be his brother-in-law.