Caoimhe won the 'Save and Replace' task and chose Mario to face eviction from the 'Big Brother' house this week along with Ife and Corin.
The Irish student burst into tears after swapping her picture for Mario's on the board of housemates nominated to face the public vote, after winning a cowboy paintball pistol duel against Corin and Ife.
After being declared the winner of the task, Caoimhe tried to go for a cigarette, saying "I can't do this", but was called back by Big Brother.
Corin told her: "If it was me and I didn't know who to choose I'd do eeny meeny miny mo."
Ife told her: "You've got to do this because it's torture for them sitting waiting over there. If it was me I'd pick someone I'd nominated, but we must both be sitting here with you."
Caoimhe said: "OK, I'm just going to go with someone I don't think is going to go. I have to go with that gut feeling. Mario I don't think you're going to go. I'm sorry."
She went and hugged Mario but he shrugged and said: "Shabby did it and now you. I knew you'd pick me."
As Caoimhe broke down in tears, John James and Dave comforted her. Corin, Ben and Keeley comforted Mario.
Caoimhe only won the task by the skin of her teeth, as the paintballs bounced off the targets.
Only one managed to burst and hit a target when Caoimhe's grazed the edge of Corin's target on her first attempt.
The three had joked during the duel task which saw each of the three nominees, dressed as cowgirls with targets tied to their chests, take it in turns to stand back to back in pairs, walk three paces and then turn and fire at each other on the sound of a bell.
Army veteran Steve, dressed as a sheriff, had to inspect the contestants' chests in between each try to see if they had hit the targets.
He joked: "Oh this is a good job. Ding dong," and Caoimhe teased back: "Ding dong, daddy."