I am not North American, I am Irish but hopefully I can give you some insight. there is a certain amount of racism in Ireland, unfortunately, but no more so than in any country that is predominantly white. What you will get, especially if you live in a disadvantaged area, is the usual gangs of kids hanging around outside shops who might pass comment as you are walking by. I live near local council flats and you do hear inappropriate remarks sometimes, but these are only bored kids and they will generally pass comment on anyone who looks different e.g. 'alright fatso', 'state of your hair' etc. Nobody pays much attention to this. I have heard the odd remark about people of other races by these types of kids on the bus the odd time but it is the rule rather than the exception and their intention is to draw attention to themselves and what idiots they are.
Ireland, especially Dublin city, is becoming increasingly more diverse in terms of race. Incidents of racism are by far the exception and hopefully something you will not experience if you come here. By and large people will chat to you in the pub and be interested to hear where you are from etc. and the vast majority of us welcome people from different countries and cultures - it definitely makes socialising (whether you drink or not) more interesting.