Some girls now require dresses with a 48-inch chest-size, and increasing numbers of boys are wearing trousers with waists of up to 38 inches.
Doris Healy, who runs Communion costume retailers The Sisters in Tallaght, said oversized outfits accounted for 20pc of her sales.
"Certainly 10 years ago, everyone could fit into regular-sized outfits and it was very rare to see a child who was obese. But things have changed dramatically since then.
"A few years ago I started to notice we were having problems, because girls were coming in and we just didn't have dresses big enough for them to fit into.
"The same was the case for boys. We didn't have big enough trousers for them.
"Every year, we have to order bigger sizes and more of them to accommodate the customers."
She said that she began noticing the trend during the boom years. Her recent customers have included an eight-year-old who weighed 12 stone.
"She was swigging from a bottle of coke and her mother promised to take her to McDonald's if she was good," she added.
It is thought that almost one in four boys (23pc) on the island is either overweight or obese. And the rates are higher in girls, with 28pc said to be overweight or obese. The problem is more severe in the Republic than in Northern Ireland.
Experts say that the reality is that, for all but a minority who have medical conditions, the pounds are piling on because of their diet and exercise levels.
The advice is that children and teenagers should be encouraged to taste everything on their plate, but they also need to be allowed to decide when they have had enough.
Last year, it emerged that one of the country's biggest chain stores was now selling plus-sized school uniforms, in response to demand from parents desperate to find a uniform to fit.
Meanwhile, it was revealed last week that that cash-strapped mothers are being forced to sell off their children's Communion outfits to make ends meet.