Niall was born in Mullingar, County Westmeath, Ireland. He is the son of Bobby Horan and Maura Gallagher. His only sibling is an older brother named Greg. Bobby and Maura divorced in 1998 when Niall was only five years old. He lived in both parents' homes before finally settling with his dad. He felt that living in Mullingar was the easiest thing to do at the time, because all of his friends were there. Niall's stepfather is from England, and he has family there.
Niall was a pupil at Coláiste Mhuire, Mullingar, a boys' Christian Brothers school. He has been playing guitar since he was around eleven years old, and performed around his homeland, including as a support act for Lloyd Daniels, in Dublin.
In 2010, at sixteen years old, Niall auditioned for the seventh series of The X Factor UK in Dublin.