The 19-year-old star took a break from One Direction's 'Take Me Home' tour - which is currently in the US - to be with his older brother Greg Horan when his sister-in-law Denise went into labor, and after hours of waiting the singer was overjoyed to finally become an uncle to the "cutest kid ever".
In a message on Instagram, accompanying a picture of him holding the baby, he wrote: "Congrats Greg and Denise! Wohoooo! I've got a nephew! Cutest kid ever! (sic)"
Niall's photo has already amassed a staggering 594,000 'likes' from One Direction fans on the social networking site, and several supporters suggested the little one had been named after Greek God Zeus.
Despite the Directioners' eagerness to give the baby a moniker, Greg and Denise are not thought to have chosen a name for their newborn just yet.
Niall discovered he was going to be an uncle for the first time when Greg and Denise told him the good news at their wedding in March.
The pop star was Greg's best man on his big day and he made sure proceedings got off to a great start by singing boy band Busted's hit song 'Crashed The Wedding' as the family drove to St. Michael's Church in County Westmeath, Ireland.
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