I remember when I first returned to Ireland, I was genuinely frightened at the amount of fights late at night anywhere you went in Ireland. Some of the bigger towns around the country especially. If you think being a Dub is bad in these places try being a Corkie!!
As others have said you can (touch wood) avoid certain areas and keep the head down but at the same time the sheer numbers coming out of clubs and pubs at the same time and the aggression that takes place can be frightening if you are not used to it or have forgotten how to deal with it or avoid it.
It took me a while to get the bearings again and now whenever I go out I leave just before the pubs and clubs start chucking out.
The biggest problem though remains everyone getting turfed out at the same time all pumped up. Let them dance til the early hours until they are knackered out!
Having said all that it doeas seem to have got better over the last few years.