After a few unpleasantnesses on the train to Holyhead and the boat, I used to make a point of checking the fixtures and trying to avoid days with potential for obnoxious fans to be about.
The worst I ever had were Australian rugby fans, who were just atrocious. Yet, on another occasion, I noticed a few matching shirts on the boat and became aware that there were dozens on fans aboard. I had to ask who they were, as it was an away shirt and I didn't recognise it - they were Wolves fans - so, you can't always tell, I would have travelled another day if I'd known they were going to be on and yet they were absolutely no trouble....