he received a message on Snapchat from Grace before she got into the car.
Grace's brother was driving her and her friends to a post-exam results celebration when the crash happened at about 7.30pm close to the entrance of Hillview Sports Club on Mountain Road.
"I looked at locations [on the Snapchat app] and I saw where her location was," Aaron said.
"Then, I got a phone call from one of my friends saying there was a crash up in Hillview. I was like, 'that's her road'.
"I ran from one side of the town over as quick as I could, but I couldn't get up because the guards had it all blocked off."
He added: "There was something in me that didn't want to believe it was their car, but I could see the car, and then I knew what it was. It was hard."
Aaron and Grace met at a party hosted by Zoey, who was also killed in the crash.
Aaron said: "Probably the first day I met Grace was one of the happiest days of my life. We just clicked. She was the love of my life."
He described Grace and her friends' close relationship, saying that the girls "together were just wild fun", adding: "You couldn't separate them."
Aaron's emotional tribute comes as a vigil is to be held to remember the four young people this evening.
Hundreds of people are expected to gather at 6.30pm in the Denis Burke Park in Clonmel.
Yesterday evening, the girls' principals paid tribute to their talented students.
Zoey and Grace had attended Presentation Secondary School in Clonmel, while Nicole attended neighbouring school, Loreto.
'SADNESS AND DEVASTATION' Stunned teachers told of how all three girls had received excellent exam results and were looking forward to embarking on the next phase of their lives.
Michael O'Loughlin, principal of Presentation Secondary School, said: "Today, our school and our town are covered in a cloud of sadness and devastation.
"This morning, we mourn the loss of our Leaving Certificate students Zoey Coffey and Grace McSweeney, who lost their lives in yesterday's tragic accident.
"We also remember Grace's brother, Luke McSweeney and Nicole Murphy, a student from our neighbouring school, Loreto.
"Zoey and Grace received their Leaving Cert results yesterday and were about to embark on a new chapter in their lives.
"A day of celebration is now replaced by unbearable grief and a sense of great loss for our local community. Our school community is in mourning today. No words can express our sorrow, pain and grief."
'BEAUTIFUL STUDENTS' Mr O'Loughlin continued: "Zoey and Grace were two beautiful students, loved by everyone who met them and achieved such excellent results on Friday."
Principal Anne McGrath, from Loreto Clonmel, described Nicole as a "highly regarded" student and a "loyal friend."
She said: "No words can express our sorrow, pain and grief. Nicole was a beautiful student, highly regarded by staff and students alike.
"During her time in Loreto, she developed into a young woman who was kind, gentle and witty, and she was a trusted and loyal friend."
The principal said the school community was struggling to come to terms with "the shocking news".
Ms McGrath continued: "Nicole achieved excellent Leaving Certificate results yesterday and was looking forward with excitement to starting the next phase of her life.
"Yesterday's celebrations are now replaced by such unbearable grief and a sense of great loss for our local community.
"In particular, we are thinking of Nicole's parents, Serena and Declan, her older sister (our past pupil) Zoe, her brother Ryan, her extended family and her close friends. We are keeping them all in our thoughts and prayers."
He described the friends' close relationship, saying that the girls "together were just wild fun, adding: 'You couldn't separate them' Credit: //www.presclonmel.com/ -Collect Aaron and Grace met at a party hosted by Zoey, who was also killed in the crash Credit: Garda press office handout Principal Anne McGrath from Loreto Clonmel described Nicole as a "highly regarded" student and a 'loyal friend' Credit: Garda press office handout 'We also remember Grace's brother, Luke McSweeney and Nicole Murphy, a student from our neighbouring school, Loreto' Credit: Garda press office handout