Shannon Airport is the main airport of Ryanair. Shannon City itself, is of no great interest, besides being the airport with the most Ryanair flights. It's 200 kilometers from Dublin, but there are direct buses that leave from the airport, you don't even have to go to the city center to catch the bus. It cost only 10 euros if you buy them online. Then there is the nearby town of Limerick. It's an industrial town but it has a nice medieval core. From Shannon, Ryanair flies to London, Paris, Poland, Belin, Girona. There are also some Aer Lingus and other Irish low cost flights, as well as flights to New York with Delta or Continental Airlines.