7/8 hours in Newark would not be fun.... Assuming you can get rid of your luggage then I'd certainly head into Manhattan - the train is in easy, relatively cheap and has you in Penn Station in the heart of midtown in under 1/2 hour
EDK: Is there anywhere one can desposit cases in Newark Airport safely?presumably having cleared customs there are only 'normal' security checks for internal flights. in relation to the airport building where is the train station located and how often do the trains run? anyone know of a site where the train timetables are posted?
Normally (and IIRC Newark is no exception) when you are arriving in the US and transferring onto a domestic flight you must retrieve your bags and clear customs ... but then immediately after customs (before you exit to the public area) you can re-check your bag at an airline desk (Maybe you have to get them tagged to the final destination in Dublin before you leave - not sure?)
So assuming you can do this then you would have no problems. I have no idea if there is a left luggage facility - given the security mania in US airports I'd doubt it but I don't know. Best to ask when you check in in Dublin to get your bags tagged to your final destination and then you can get rid of them once you've cleared customs.
The train is located upstairs in Newark - it's a monorail which links all the terminals and parking lots and ends up at the NJTransit Railway Station (about 1 mile from the airport) - There are frequent NJ Transit trains into town then - at least every 15/20 mins and they only take about 15 mins to get to Penn Station - total journey time including the monorail is around 30 mins.