I'm currently employed as a U.S. citizen in the USA. However, I have dual Irish/US citizenship (and both passports), as my father was born and raised in Cork. I'd like to request a transfer to the UK from my company (as US subsidiary of a British company). Do I need a work visa (since I'm currently employed as a U.S. citizen) or can I just work there as an Irish citizen (due to EEA recognition)? (If I get the transfer, I will work with an immigration attorney to make sure we do it right, but I'd like to "sell" the transfer to them as something easier/cheaper/faster to do if I don't have to get a work visa.) Thanks!
You do not need a work visa. You are an Irish citizen, and Irish citizens do not need authorization to work in the UK. The fact that you are also a US citizen who is currently working in the US does not change that.
You also do not need an immigration attorney. All you need is an Irish passport.
You may want to consult with a financial advisor, however, since the US will continue to require you to report your worldwide income and figure tax on it, though there is an exemption for foreign earned income and for taxes paid to other jurisdictions. You'll also want to be aware of FATCA.