It was a bank holiday in England when we went to Galway and there seemed to be a gathering of Stag and Hen parties from all over the UK the 24 hours we were there.
This is what I was able to surmise from the few hours I was privy to this world:
1. Hen parties are license to dress slutty and must always contain a bevy of feather boas. Usually pink. Pink Cowboy hats are another requirement.
2. Stag parties let the inner nerd that these men barely hold in on a daily basis be fully expressed.
We saw these guys later hanging out drunk before noon with a couple of Magnum P.I.s. Those guys were smart enough to go with a stud-ish costume. Chewbacca would not be getting laid. I have a feeling his wife suggested that costume.
Full Disclosure: I made a Princess Leia costume (from Episode 4-not the naughty one) 12 years ago and wore it 3 Halloweens in a row. I too am a nerd.