This month the pair completed the mammoth event, despite Sharn's rapidly deteriorating condition.
One year ago, the 31-year-old's was diagnosed and given a maximum of five years to live. It was news that devastated the pediatric nurse and her husband Russell.
But it wouldn’t detroy her dream of completing the ultimate physical challenge.
"If you can approach any life challenge with the right attitude then I think it makes for a better world," Sharn told Sunday Night reporter Denham Hitchcock.
It was Russell's best friend Craig Gruber, Sharn's training partner, who first noticed her symptoms
"We got back from the ride and Sharn said her foot had gone to sleep, she was sort of limping," Craig said.
Sharn eventually went to see a specialist at Sydney’s Saint George hospital, where she works in the children’s emergency ward.
"I noticed that I was getting a bit of a foot drop when I was running and the muscles weren’t quite working right and I thought 'oh must be muscular'," Sharn said.
After eight months of tests, she was diagnosed with a strain of motor neurone disease, present in less than 2000 Australians.
There is no treatment and no cure.
"It has been known as the disease where you basically watch yourself die. The final stage really comes with respiratory failure… That’s usually the end of the story," Sharn said.
It was after her diagnosis that Craig Gruber approached her with the idea to complete the Ironman, with his help.
"I want to do the Ironman with you… if we train right I will tow you in the swim, only 3.8km, I will pull you in the ride, just a short 180km and I will run and push you in a marathon, 42km."
"I just said look I want to take you around Ironman, I want to give you the chance to give you your finisher’s medal," Craig said.
They trained together almost daily despite Sharn's muscular deterioration and, eventually, made it to the Cairns Ironman.
The swim and bike legs took more than 10 hours — Craig then changed from pulling to pushing Sharn and together they completed the 42km marathon as darkness fell.
They said they couldn't have completed without the incredible support on the sidelines.
"Without that sea of pink and blue and white there was no way we could have done this. Every time we needed them they were there for us, it was something else," Craig said.
It took 16 hours and 24 minutes to get there, but Sharn got up and insisted she walk the last 100 metres.
"I knew from the beginning I wanted to walk in whatever capacity I could,"
"I just felt so happy for Sharn… it was oozing out of me, my happiness for her." Craig said.
Sharn hopes her story helps create awareness for the disease to eventually find a cure.
"That's my new challenge. We need to find a cure and I'm going to make it happen, just like I did the Ironman."
Watch the full video of Sharn's story and the race above.
You can make donations to help Sharn and Russ with medical costs and also help her access a new experimental treatment in Brazil that could lead to a cure for others. Go to www.shining4sharn.com.au
For information and support please contact the MND Association in your state or territory on 1800 777 175 or visit the MND Australia website on www.mndaust.asn.au