The 18-year-old from Artane attended the gig with a friend to celebrate her milestone birthday.
Speaking to sundayworld.com Alex revealed how she managed to grab Harry’s attention.
“I brought a sign with me that said ‘today is my 18th birthday’,” she explained.
“He just looked at it and came over and he was talking to me, [he asked] ‘what’s your name?’ and ‘where are you from?’”
“And then he asked everyone if they wanted to sing me Happy Birthday, and then they sang Happy Birthday to me.”
The teen and her friend were left in complete shock when the almost 52,000 attendees began to sing for her.
“The whole stadium sang Happy Birthday.”
“I didn’t think it was going to happen,” she said.
“I wasn’t expecting to be as close to the stage as I was. I was just shocked.”
“Everyone is shocked. My friend that was there with me was like ‘that just happened!’ and I was like ‘I know, I can’t believe it.’”
Alex was not the only fan to get the former One Direction stars attention.
Another fan held up a sign reading: ‘Tallaght House Party?’
“I've been to Tallaght lots,” Styles said.
“I got headbutted. It was great. It was fantastic! But I did buy..... 2 bananas for a pound, 3 bananas for a euro!”
The sign was in reference to a moment during a One Direction show in 2015 where Harry and Irish bandmate Niall Horan recalled a party in the Dublin suburb.
During the One Direction show seven years ago, Harry asked the crowd: “Is there anyone here from Tallaght, I'm going to tell you a story.”
“About five years ago I travelled over to Ireland with young Niall, we went to Mullingar, and we went to another party, didn't we Niall,” he said.
Asking Niall where the party was, Horan replied, ‘Tallaght’, to rapturous applause from the audience. Styles went to ask the Mullingar native, “what happened to me in Tallaght?”
Horan replied to by saying, “Some fella stuck you a headbutt”.
Styles added: “I went to Tallaght one time and got the nut stuck on me, but luckily for him he managed to run away quick, I could have had him if I wanted to but I’m a lover I’m not a fighter”.