Having a FAA license, based on a foreign license, always restricts you to the "most restricting" license. I.e. even though you have a FAA PPL, you can not fly at night, if you don't have a nightrating on your foreign license. BUT also the other way around as well, you will need to abide by all the FAA regs as well as the Irish ones.
A flight review, or something that counts as a flight review (passing a checkride for instance) is required every 24 months to be able to legally fly. Since you have not passed a checkride, you have nothing that counts as a flight review. You will need to do a flight review.
As far as the flight review goes, study might be required.
Every instructor is free as to what to ask in a flight review, but expect the Q&A portion to be alot about airspaces and associated rules and regulations, and regulations in general. But remember the instructor is free to ask what ever he/she thinks is required for you to be safe. There are study books specifically for the flight review if you want to brush up on things. Or go talk to an instructor, they might be able to tell you what they would require you to know before they would sign you off.
Personally, i take a map, and take them through a flight, asking questions about airspace along the way. (example: You are doing a preflight, what papers are you looking for? What papers will you bring yourself? You are departing this airport, what airspace is it? what communication is required? You are now flying here, this altitude, what airspace is it, and what are the VFR minimums? you want to go through this portion of special use airspace can you do that? you want to land at that airport, what are the requirements to get in that airspace, etc etc.....)
I usually ask a little bit about communications (using the map, and who to call when) since it is a little different as in europe. Usually some airplane related questions, and for the rest it depends on what i think that student is lacking in knowledge. I am not after failing people for that, I just want to make sure that people are safe, so if you don't know one or two questions, don't worry.
As far as your medical goes; If your PPL requires you to have a irish medical before it is valid, you will need to have a valid irish medical. If your irish license is valid, even without a irish medical, you can technically also get a FAA 3rd class medical (or higher of course), and fly on that. But since you said you have a valid irish medical, that is not a problem for you.
-IBLB-