The Donegal beauty admits she had no idea what she was letting herself in for when she agreed to face off against Happy Pear twins David and Stephen Flynn, comedian Neil Delamere and children's TV presenter Sinéad Quinlan.
"I actually thought it would be really good fun and I was trying to get my sisters to do it with me and they were like 'Gráinne, it's going to be really hard,' and I was 'no it's just a celebrity version, so they're not going to put you through the paces - it will be great, we should do it all together'," explains former Miss Universe Ireland winner Gráinne (27).
Her boyfriend, Ryan Coleman, also declined to take part.
"He said he would have loved to have done it and it would be good fun, but he said 'I couldn't do the whole TV aspect of it'.
"They all wouldn't do it, so I got my brother-in- law Dee Foley and two of my cousins, Chris O'Donnell and David Gallanagh, to do with me," she says.
Gráinne was in for a massive surprise, though. "My sisters were right - it was horrific," she exclaims. "It was so hard, because they really didn't take it easy. It was the same course that they obviously use for Ireland's Fittest Family
"I feel like I was really taken by surprise at how hard it was. I just spent most of my time being dragged around the courses by my cousins. Thank God they were so good, because I was like a dead weight to be honest."
Gráinne has terrible flashbacks.
"There was one course involving heights that we done and I just felt at that point, I was so exhausted," she recalls. "I was just terrified of doing it as I hate heights. Before we did that, I could feel myself tearing up, it was so dramatic, but I just felt like I didn't think I was going to be able to do it and I didn't want to let anybody down."
She took part in Dancing With The Stars last year with Kai Widdrington.
"It was so different," she reflects.
"It was every day and you sort of build up momentum. Obviously, it was completely different, and it was so hard."
However,she keeps herself fit.
"I love going to the gym and loved taking care of myself, more so just for my head, I find it helps me."
She also tuned in to see Kai taking part on Strictly Come Dancing, before he was forced out of the final after his partner AJ Odudu got a foot injury.
"He had wanted to go on Strictly for so long and it was just his dream," she notes. "He did amazingly well, he was so brilliant. I felt really bad for them, they were devastated but brilliant."
Grainne works as a nurse, mainly in a clinic near her home in Buncrana.