Just how did an Irish start-up land investment from a member of one of the biggest boy bands in history? For Niall Horgan, co-founder and CEO of Irish athleisure brand Gym+Coffee, the answer was a mix of luck and pluck. Last October, the fast-growing Irish business was preparing to open its first UK store in London. Horgan and his team wanted to create a buzz and sent out invites to as many high-profile people as they could to attend the launch. One of those on their target list was Niall Horan, the Mullingar-born singer-songwriter who first found fame as a…