The 18-year-old was knocked out at Judges' Houses by Kelly Rowland last year and was also dumped out in 2008 by Cheryl Cole, who thought the singer was too young for the competition.
McCabe stuns the judges with a performance of 'Bridge Over Troubled Water' this evening, which Gary Barlow describes as "extraordinary".
Louis Walsh comments: "I'm emotional because I know you're from Dublin, what people don't know is that we know each other and I know you have been through a really tough time.
"I didn't know you were going to be this good today, I knew you were going to be good but you were incredible and I am so happy for you."
Guest judge Mel B adds: "Well, I didn't know what to expect, you're this tiny little petite thing with this huge incredible voice, I was like, 'Woah, where did that come from?' You're a little star in my eyes."
Tulisa says: "Melanie, your voice is even better, than what I remember from last year. That performance was outstanding, the raw talent that you have is just undeniable, you deserve to be on that stage and I'm so happy that you are there."
Speaking to the cameras and Dermot O'Leary before her audition, Melanie reveals: "I was so excited last year getting to [Judges' Houses in] Miami, I'm from a little town where it's kind of like just unheard of, you know?
"When I was told that I was going home, I was devastated. I couldn't even talk to anyone, I came home and my mum just cuddled me and she told me it was going to be okay and I still can't watch that Judges' Houses episode, it just brings back so many memories.
"I was so devastated when I didn't get through, I cried the whole way home from Miami. I think in the past year I've just kind of said to myself, 'No, I'm going to come back this year, I'm going to do better'."
The X Factor, which includes Melanie's audition, airs tonight at 8.10pm on ITV1.
Photo gallery - The X Factor show four:
Copyright: ITV Thames/Syco
Watch Melanie's Judges' Houses performance on last year's show: