Niall Horan will be his brother's best man at his wedding.The One Direction star is set to return to his home town of Mullingar in Ireland for his elder sibling Greg's impending nuptials to fiancée Denise, which are set to take place in the next few days, and his bandmates have also been invited.A source told The Sun newspaper: "Niall's excited for his best man duties. He's been writing a speech and made sure he had time off from touring for the big day."After the ceremony Niall's thinking about getting up on stage with his guitar and singing at the reception."Niall's brother is thought to have hired extra security for the wedding in anticipation of excited fans hoping to catch a glimpse of the 19-year-old singer.Meanwhile, the 'Kiss You' hitmaker sent fans into a frenzy on Twitter yesterday (24.03.13) as a shirtless picture of the star showing off his recently toned physique circulated amongst fans.One tweeter said: "Niall Horan shirtless will be the death of me."Niall also apologised to his fans for his absence on the social networking site in recent days, tweeting: "I finally have signal! I've had no phone signal all week! So annoying!"