The 63-year-old former supermarket boss was a sizeable 18 stone 10lbs when his doctor told him about the health risks he was facing.
Mr Dunne took the advice on board and immediately went on a diet, limiting himself to between 1,500 and 1,800 calories a day.
The businessman, who owns a string of leisure centres in Dublin, also added more exercise to his routine.
"I've lost four stone after taking medical advice," Mr Dunne told the Herald today.
"When you get into your 60s and when you see your consultant and he says it's advisable if you want to live into your 70s to lose weight, then you take notice of your consultant," he said.
At his last yearly check-up in January, Mr Dunne was told he was "perfectly healthy" but the doctor was not so sure if he would remain in good shape in years to come.
"When you're given the real facts by a top consultant you take them on board very quickly -- when you are heading for 63 and he is saying you might not see 73," he added.
Such has been the transformation in the intervening months that the businessman has gone unrecognised by some people.
"I watch what I eat but I don't worry about weekends.
"I go out Saturday night and have a meal and on Sunday I would have lunch," Mr Dunne told the Herald.
"I exercise in my own gym and I walk," he added.
"Loads of people have said to me 'could you not give me your diet menu' but the funny thing about diets is that what works for one person does not work for someone else," Mr Dunne said.
Mr Dunne also said that he has not found it difficult and is just thankful he got enough notice to allow him reform his eating habits.
One consequence of his lighter frame is that he has had to change his wardrobe.
"I had to get rid of all my clothes. I gave them all away and turned over a new chapter," Mr Dunne said.
Among the items he has ditched from his diet is sliced white bread and sweets.
"I've not eaten chocolate since January. I eat a lot of vegetables, fruit and protein," he revealed.
His weight now varies between 14 stone 12lbs and 15 stone. Asked about what difference it has made, he said: "You've no idea."