Conor Devine was diagnosed with multiple sclerosis (MS) in 2007.
MS causes the body’s own immune system to attack the lining of nerves in the brain and spinal cord.
When this happens a person will have symptoms such as fatigue, blurred vision and they can have difficulty walking.
The 41-year-old says he is fighting back against his illness having already completed 10 marathons and two Ironman competitions.
An Ironman consists of a 2.4 mile swim, a 112 mile bike ride and a 26.2 mile run.
On Sunday he will take part in his third Ironman, when Ireland’s first full-distance event takes place in Cork.
Video journalist: Niall McCracken