And he warns that what happens on the faraway streets of a Philippine shanty town will eventually impact on small-town Ireland.
"The abusers do not leave their habits and desires at the airport. These paedophiles come to whet their appetite, have their fill and return with their desires to molest children greater than ever," he says.
The sordid 'industry' in the Philippines involves everything from child slavery to children as young as 10 being sold as sex slaves.
Fr Cullen, who was nominated three times for the Nobel Peace Prize, has been fighting the Philippine authorities for 40 years to stamp out the business. Now, after much success, he finds himself confronting Irish sex tourists on a regular basis.
"There are sex tourists from Ireland, we don't know how many - but they are here - we bump into them from time to time. You have lots of paedophiles who hide out here: Irish guys, German, Swedish."
According to the priest, the South-east Asian tsunami wiped out hundreds of sex resorts. The sex tourists are now flocking to the Philippines, which was spared.
Angeles City, which is two hours' drive from Manila, is the home of the Philippines sex industry. Thousands of sex tourists from all over the world, including Ireland, visit the place every week in search of cheap sex.
According to Fr Cullen, many of the tourists go in search of children younger than 12, of which there is a plentiful supply.
"Fourteen-year-old girls living with 70-year-old foreign men as sex comfort girls is so common that the government officials wonder what all the fuss is about," Father Cullen says.