The Green Party is putting pressure on Finance Minister Brian Lenihan to impose a 1c 'tax on texts' on all mobile phone users.
But they have got their sums wrong on how much it would generate. In fact, they are more than €1bn out on how much revenue it would raise.
The junior coalition partners said the plan is a "creative way" of tackling the new €5bn deficit in the public finances in the supplementary Budget next month.
Deputy leader Mary White estimates that a 1c tax on every text message could raise €1.4bn for the Exchequer.
"In tough economic times, it seems a fairly equitable way of getting revenue in quickly without inflicting too much pain," she said.
The tax would apply to all 25 million text messages sent by Irish mobile phone users every day, or more than eight billion texts every year. But rather than the €1.4bn that Ms White has claimed the move would raise, the real figure would be in the region of €81m.
The deputy said people were free to disagree with her calculations, based on the country's four million people sending an average of 10 texts a day each. But even this only adds up to €146m. Ms White has been discussing the plan with Communications Minister Eamon Ryan since last October and hopes to present it shortly to Mr Lenihan after a parliamentary party discussion.
"It's an idea that may be trashed or implemented, but we'll see if we can put something together and put it to the Minister for Finance," she said.
Ms White admitted her tax on texting would apply to children who use text messages as a low cost way of keeping in touch. But she said it would be parents who would be footing the bill.
"As a mother myself, I know children love to have their mobile phones and they're very important in terms of knowing where they are to keep them safe," said the Carlow/Kilkenny TD.
"But it might be no harm if they were aware of a 1c tax on texts -- prudent economy at an early age."
But the Irish Cellular Industry Association, which represents major network operators such as Vodafone and O2, said it opposes any tax on texting.
Its director, Tommy McCabe, said mobile phone users already pay VAT at 21.5pc on phone calls and text messages.
"This would be double taxation. And a lot of text messages are part of bundled packages where you get free texts, so it would be difficult to implement," he said. Mr McCabe also said text messages were used mostly by "the most vulnerable", teenagers and children, because they were less expensive than phone calls.
Fine Gael communications spokesman Simon Coveney said his party would keep an "open mind" on the Greens' proposal to tax text messages as "we need to consider all options".