A professor of economics has said water charges will have to be considered in Ireland at some point.
Prof John Fitzgerald is adjunct professor in economics at Trinity College Dublin (TCD).
He was responding to a report, which has suggested the issue needs to be looked at again.
The Organisation for Economic Co-operation and Development (OECD) said Ireland may need to introduce water charges among a range of measures it believes Ireland needs to meet its environmental targets.
But both the Taoiseach Micheál Martin and Environment Minister Eamon Ryan have ruled it out.
"No, we won't be going back on that - we won't be re-introducing water charges", Mr Martin said.
"We will not be going back to the water charges issue - I think it was agreed in the Oireachtas all-party committee", Mr Ryan added.
In 2014, tens of thousands of people took to the streets when the then-government planned to introduce water charges. The plans later had to be abandoned following the public backlash.
But Prof Fitzgerald told Pat Kenny that at some stage, water charges will have to be looked at.
"I think that tackling climate change is definitely going to cost us.
"Joe Biden likes to talk, Boris Johnson likes to talk about all the jobs - yeah, but is actually going to cost us something.
"It is something we've got to do, and we have to minimise the cost of doing it".
"Every drop of water we use costs money, because it's been cleaned and so on - so we need to pay for it.
"But one of the problems is that Irish Water doesn't have the funding.
"It's not like the ESB [that] it can go off and invest what's needed to make the system work.
"It has to rely on money from the Government.
"Whereas if it had it's own source of income, it could actually invest much more and it would not add to the national debt and so on.
"The issue on water is a) if you charge, people don't waste water - which is really important - but it also gives a flow of income to Irish Water so they can do the job properly, which they're not able to do at the moment".
While speaking on Monday, People Before Profit TD Paul Murphy said any new attempt to introduce water charges will bring tens of thousands of people back out onto the streets.