Accompanied by the bodhrán, Andrea, who is based in England, performed U2's With or Without You for the Judges and won the last free seat on stage in the Over 25s category.
After her audition Gary Barlow had high praise for the music teacher, telling his fellow panelists: "She's different from everybody else."
But while Andrea was given a place in the final six, she had a torturous wait to find out whether she was safe, as many more performers still had to take to the stage.
This year's Bootcamp saw the introduction of the seat challenge where each mentor could give a spot to six of their favourite acts. If, when all six seats were filled, they preferred another act, they could still be put through but it meant one of the chosen six has to be replaced.
While Andrea managed to keep her seat, fan favourite Joseph Whelan was on the verge of being swapped out after final performer Zoe Devlin took to the stage.
Following her rendition of Joe Cocker's You Are So Beautiful, Sharon told her: "You are very different to the singers that are already in the seats - I want you to have a seat."
But Sharon had difficulty deciding which act to replace Zoe with and asked her fellow Judges for help.
Gary told her he would swap out Joseph, causing the emotional singer to jump up in disbelief.
After Sharon ordered him to return to his seat, she decided he would not be replaced and told Zoe to take Katie Markham's spot instead.
That was a beautiful song choice and it was really beautifully sung.
Gary Barlow to Zoe Devlin
Also winning a place in the Judges' Houses stage of the competition were prison guard Sam Bailey, white van driver Shelley Smith and soul singer Lorna Simpson.
The drama continued for the Groups category as trio Rough Copy had the heartbreaking decision of letting go of one of its members.
The group made it to the Judges' Houses stage in 2012, but had to withdraw from the competition because of visa issues.
This year the problem arose again for one member of the group, but rather than all three members losing out, they decided to continue as a duo.
The pair won a place in the final six after an emotional performance of Oasis' 'Stop Crying your Heart Out'.
Sharon told them: "Rough Copy have had a rough time of this, but you are here and you gave a great performance."
Joining Rough Copy in the final six of Gary's Groups category are boyband Kingsland, slick four-piece Code 4, vocal group Brick City, girlband Xyra and a new girl group which Gary put together at Bootcamp.
Meanwhile Louis Walsh chose his final six Boys, including Sam Callahan, Ryan Mathie, Paul Akister Nicholas McDonald, Giles Potter and Luke Friend.
The 24 finalists will now go to their mentor's home to compete for a place in the live shows.
With crucial decisions to be made, each Judge will have help from industry figures who know all about what it takes to have a number one hit.
Gary is joined by chart topper and X Factor alumni Olly Murs, Sharon enlists international superstar Robbie Williams and Louis is joined by former All Saints member Nicole Appleton, Westlife's Shane Filan and Sinitta.
The X Factor returns next Saturday and Sunday on UTV.