The One Direction star is besotted with his brother Greg and sister-in-law Denise's three-month-old boy Theo and has treated him to all manner of gifts, including a fancy crib.
A source told The Sun newspaper: "Niall dotes on Theo and sent gifts home for him while on tour.
"He surprised his brother Greg and Denise with a beautiful crib for Theo soon after he was born and loves spoiling him."
Niall has just finished One Direction's "Take Me Home" World Tour and has headed back to the UK. He will be in Mullingar, Co Westmeath for nephew Theo's christening on Saturday.
Shortly after Theo was born in July, Niall shared pictures of him holding him on Instagram and congratulated the new parents.
He wrote: "Congrats Greg and Denise! Wohoooo! I've got a nephew! Cutest kid ever! (sic)"
Under another photo, he added: "Remember this little fella! Theo Horan ladies and gents!"
Niall and his 1D bandmates - Harry Styles, Zayn Malik, Louis Tomlinson and Liam Payne - were mobbed as they arrived back in London earlier this week, following their mammoth 2013 tour, which saw them perform more than 60 dates in Europe, 41 in North America, 25 in Australia and finishing with two shows in Japan.