I can't narrow it down to just one song. Some of my favorites are Castle of Dromore, Carrickfergus, The Foggy Dew (the one about the Easter Rebellion), Follow Me down (or up?) to Carlow, and She Moved through the Fair. There sare lots and lots of Irish tunes without words, as far as I know, which I love, including anything by O'Carolan, Farewell to Erin, Dennis Murphy's Polka, John Ryan's Polka, Bill Sullivan's Polka (I hope I got all those polka names right), Kesh Jig, Morrison's Jig, and many, many more.