From what I remember, they had been broken up for months and then decided to give it another go. They were back together 2 weeks and he cheated for the 7th time so she ended it again.
Charleen was in New York with her friend and Ellie flew out to her to get away from Conor. He followed her to try win her back.
He was there less than 48 hours, I think, and Ellie rejected him so he flew home but vowed to win her back but now, through his slip up, it seems he was already with/talking to Aoife at that time.
I'm open to correction on that though because they seemed to break up and get back together so many times