The type of person referred to ad a "scumbag" in Dublni, a "ned" in Scotland, and "BAD NEWS" the world over.
Mouthy youths with big 'toods, even bigger knives and a record as long as your arm.
There are some very bad neighbourhoods in the city center,where you wouldn't expect. The entire central area if full of housing projects. With the historical stuff and new developments mixed up in it.
You go two blocks the wrong way and you WILL get robbed. DO not get into a face-off with a drunk person for any reason, and ignore anyone who shouts insults at you, they are doing it to have an excuse for a fight, and to a Dublin "scumbag", a fight means knives and blood, not boxing.
Make sure you get some advice from Irish folks about the areas to stay out of. The Holiday Inn is in a SHIT part of town, don't stay there. Likewise, to go from the centre to the Guiness brewery, well worth the visit, you go through an area known for being a good palce to score some herion (not a neroine!).
Ireland was very poor for a long time and there's still a big drugs and violence problem in Dublin. However, outside localised neighbourhoods there is very little trouble.
Watch your handbag, mainly, and don't get talked into a fight. If someone pulls a knief or yringe and demands your bag, give it to them.
You probably won't have any trouble at all anyway.