The Catholic Church has hit out at what it calls Mass card trafficking.
Mass cards with printed signatures are being sold
It says Mass cards with printed signatures are being sold in some shops, with Catholics believing the cost is the offering for the Mass.
Instead, it is claimed most of the money goes to the shop and distributor, with very little going to the church.
Bishop O'Reilly from Longford said it was a massive industry and the lack of accountability undermined public confidence in missionary work.
He said missionary orders were concerned people were buying these Mass cards thinking the money was going to support charity work in the third world.
In the past, people bought an unsigned card and got local priests to sign them, while making a donation.
"People that want to make Mass offerings should do so on a personal basis with an individual priest," said Bishop O'Reilly.
"I would much prefer if they wrote a letter to the person who's been bereaved or better still if they undertook to personally fulfill some act of charity or prayer," he said.
The Irish government has looked at this issue when tightening up its charities' legislation.
Fine Gael TD Michael Ring claimed that in the case of some Mass cards being sold from retail outlets, the names of priests "who had been dead for years" had been used on them.
"In other cases, writing was illegible and they were not able to discern who the priest was," the Mayo deputy claimed.