Eleven local authorities across the country have more people on their election register than the population that is eligible to vote.
An Coimisiún Toghcháin, the independent electoral commission, said it is deeply concerned at the legacy accuracy issues surrounding the multiple electoral databases - following a significant research project.
There are currently 31 electoral registers, the vast majority of which do not "talk to one another", meaning duplication is common.
Some political scientists have estimated that up to 500,000 names are on the register that should not be, but an Coimisiún Toghcháin claims there is no way of accurately assessing that.
Electoral watchdog 'deeply concerned' over duplications
However, it expects there are hundreds of thousands of additional names across the registers.
It said some local authorities have a lot of duplicate or redundant or deceased names on the list, but others do not.
All of the 11 local authorities with too many people on the register are seen as having "below average accuracy indicators."
Eight of these have the lowest accuracy indicators in the country and are: Sligo, Donegal, Galway County, Cork County, Carlow, Cavan and Mayo (in order of over-registration).
The Department of Housing and Local Government is compiling a single national register, which is due to be completed in autumn 2026.
An Coimisúin Toghcháin chief Art O'Leary said that "difficulties" arise because there are 31 separate electoral registers.
"Every local authority has its own register and aside from the four [councils] in Dublin, they don't talk to each other, and I think this is where the difficulties are," he said.
Speaking on RTÉ's News at One, Mr O'Leary said that an Coimisúin Toghcháin has requested "unique identifiers" from councils for those on their registers, including PPS numbers, dates of birth, and Eircodes.
This will help it to "recognise duplicates" on the system and the maintenance and management of electoral registers "should be much better".
He also said an Coimisúin Toghcháin needs to look back to try to "clean" the data it has by encouraging people to go onto the register and update their details if they can.
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Election turnout figures 'definitely wrong' - O'Leary
The inaccuracy of the electoral registers means that official voter turnout at recent elections is "definitely wrong", according to Mr O'Leary.
"Turnout is definitely understated, there's no doubt about it, it's not possible to say how many duplicates or people on the register that there are.
"There are people who have moved address and are duplicates, there's also people who have died, people who have left the country, or are not entitled to vote, but they remain on the register," he said.
Mr O’Leary suggested that turnout at elections may be 5% higher than official figures because of the electoral register.
"I think the strongest recommendation is the need for sustained long-term investment in the register...the maintenance and management of the register carries on forever."
Mr O’Leary said that the "accuracy of the register is absolutely fundamental", adding that the creation of a single database would have happened sooner but for the number of elections in the last "couple of years".
It is a question of priorities for local councils, he said, but they are "all very busy".
"We don't it make it that easy to take people off the register because it's a very fundamental thing, the right to vote and we need to be really sure that someone is not entitled to vote before they take them off," he said.
However, it can be done, Mr O’Leary said, it just needs to be a bigger priority across the board in every single local authority.
"What we're keen to see is real progress being made in 2027 and 2028 to ensure that by the time the next electoral cycle in '29 happens our electoral registers should be in a much better place."
