The One Direction heart throb is reportedly keen to "live it up" and enjoy his youth, leading to the couple's decision to break up after only 27 days together.
Speaking to The Sun on Sunday newspaper, a source said: "It's all over between Niall and Ellie.
"They've split up. There was obviously a real attraction - but Niall's a young lad and wants to live it up."
The pair were first seen getting close at Virgin Media's V Festival in the UK last month, and Ellie later refused to open up about their then-blossoming romance.
She previously insisted: "I just can't talk about Niall. I won't ever talk about my love life any more because it's always then gone wrong when I have in the past."
The 26-year-old singer also admitted she had received some "interesting" messages from the 'Best Song Ever' hitmaker's loyal fans.
She added: "I've definitely had some 'interesting' tweets since the story came out.
"Thankfully I'm able to take it all with a pinch of salt because I've been told to be that way.
"I genuinely mean this - I don't hate on anyone, but at least One Direction fans are very passionate and my fans are very passionate too. I'd hate it if people didn't care about what they thought."