It is quite possible that you may not be able to check in until 3 hours before check-in but in that case, take Steve's advice and store in left luggage.
After that my advice would be to take the AirCoach into Dublin. It's a blue coach available from outside T2. It is more expensive than Dublin Bus but it is quicker and has no stops till it gets into the city (book a return ticket). Get off at Trinity College. From there you can visit Trinity College and the Book of Kells, go down Grafton Street (which is pedestrianised) to Stephen's Green, see Government Buildings (including Dáil Eireann, our parliament), enjoy the buskers, go down Dame Street to Dublin Castle, St Patrick's Cathedral and Christchurch, go to the Guinness Storehouse and have a free pint of Guinness!, wander through Temple Bar, go on a hop-on/hop off bus and then get an Aircoach back to the airport from O'Connell Street.
Enjoy!