I'm a U.S. citizen working in Dublin. Non-EEA nationals need a work permit - the employer is responsible for submitting the paperwork. The permit has to be approved by the Department of Enterprise, Trade and Employment before you can start working, and you must have the original form with you when you enter the country - not a reproduction. It took about 8-10 weeks for my application to be processed; I've heard that's typical.
I haven't heard of ancestry-based work visas, but I'm certainly not an expert on this subject. I believe foreign students in Irish universities can get a 4-month work permit or something.
Dublin is a terrific city. Maybe we can host our own OpenACS social some day!