The "-igan" ending

Cardigan and hooligan probably come from the Welsh name Ceredigion and the Irish name Houlighan, respectively. The Welsh name Ceredigion means "Ceredig's land". There are a lot of Irish names that end in -gan, -han or -ghan. If we go by the etymologies I can find for Monaghan and Mulligan the -an is a diminuitive. So these two are not that closely related, but mulligan and hooligan may be, since they're both from Irish names ending with the same diminuitive.

I should point out that the Irish name O Huallachain can be spelled Houligan, Houlihan, or Houlighan in English, so these three suffixes indeed have the same origin.