Horan was best man as Greg and Denise Kelly were married in the village of Castletown Geoghegan outside Mullingar.
The Irish Independent reports that a hundred teenage fans converged on the Church to catch a glimpse of the star.
The paper says there was pandemonium when 19-year-old Niall pulled into the car park at St Michael’s Church.
The singer drove his 26-year-old brother and father Bobby to the church in his black Range Rover.
Dressed in a grey suit with a pink and white striped tie, Horan signed a few autographs when he arrived and greeted fans as he left.
But the paper says he was insistent that the day remain about Greg and Denise.
A team of security guards were on duty to ensure no uninvited guests gained access to the chapel.
The only incident saw a teenage girl climb into Niall’s unlocked car before the ceremony began.
The bride’s mum Maura Gallagher told reporters that she couldn’t be happier to see her first child walk down the aisle.
She said: “The wedding was a bit emotional for me. All I was doing was staring at Greg. He doesn’t mind being upstaged at all – he’s so proud of Niall. It’s nice for Greg to have a bit of a fuss for a change.”
Local pub owner Paul Claffey told the Irish Independent that a ‘circus’ arrives whenever Niall returns to his hometown.
Claffey said: “It’s always madness when Niall’s around, but he’s a great lad and they’re a lovely family. A niece of mine actually came up from Dublin to see him.
“He’s a huge attraction, but we’re all very supportive and delighted for him – and of course the couple too.”
The newlyweds and their 150 guests enjoyed a reception at Lough Rynn Castle in Co Leitrim.
Rugby star Brian O’Driscoll and Amy Huberman held their wedding at the same hotel in July 2010.