Try the various Airline Websites - at least to get prices, some are fussy about only taking US issued credit cards. I'm sure Aer Lingus.com will probalby sell you a ticket no problem but I don't think they serve Baltimore anymore (it was on/off for a while but I think it's off for the moment). USairways.com or Contintenal.com you shoudl try also.
Even if the website won't sell you the ticket as you don't have a US credit card (they may/may not do I'm not sure) then call their office in Dublin they should do sell you the ticket no problem ... It used to be the case that there were a lot of rules regarding where tickets were purchased but I believe most of these have recently been lifted so you should not be penalised for buying a US orginating ticket when you are not in the US (years ago it definintely would have cost you extra)