Anti-Austerity Alliance TD Paul Murphy says water charge protesters are on the verge of a 'big victory', amid estimates that 30% of households are yet to register.
Irish Water is refusing to release full details on how many people have paid the charge so far, with just six days left until applications for the conservation grant close.
Those who do not register will not receive the €100 grant from the Department of Social Protection.
Irish Water has admitted that more than 500,000 households have yet to register with the utility.
It says 1.32 million households have registered their details, but it believes that is only around seven out of every 10 households.
Head of communications with Irish Water, Elizabeth Arnett, said yesterday: "All households across the country are entitled to the government €100 Water Conservation grant – even those who are part of a group water scheme or have a well and a septic tank and are therefore not customers of Irish Water."
"We are urging all households to register with us so that they will not miss out on this €100 government grant in September", she added.
Deputy Murphy told Newstalk Breakfast the campaign against the water charge has been a success.