Farrelly was born in Cabra, Dublin and was raised within the city. His father, Mark, was an amateur bodybuilder. Farrelly speaks fluent Gaelic, having attended Scoil Caoimhin Primary and Secondary School, a Gaelscoil During his school years he sang in the Palestrina Choir until the age of thirteen, as part of which he appeared on the Late Late Show and Live at Three television programmes. He played Gaelic football for the Erin's Isle team where he was declared sports star of the month. He also played rugby for the National College of Ireland, where he gained a National Diploma.