Experts warn that untreated sites can leak toxic leachate or build up landfill gas, posing risks to nearby streams, rivers and ecosystems, and potentially affecting drinking-water quality.
Ten historic landfill sites in Kildare remain uncertified by the Environmental Protection Agency - part of a nationwide backlog.
Kildare County Council has submitted just 38% of its old landfill sites for EPA certification, a process required under regulations introduced 15 years ago to ensure decades-old waste does not contaminate surrounding land, rivers or groundwater.
Nationally, just 10% of landfills have been certified.
Ireland has more than 260 uncertified historic landfills, including 87 classed as high or moderate risk.
Some have since been redeveloped into parks, pitches and public amenities.
Local authorities, according to The Journal Investigates, say limited staffing and funding are slowing submissions, while the EPA reports that many applications are incomplete, below standard or later appealed - causing long delays.
Fourteen sites nationwide have been awaiting decisions for over a decade.
In Kildare, Digby Bridge in Barrettstown, a high-risk site, is one of the few certified and actively managed sites.
But other high-risk locations are still stuck in the system, including the Knocknagarm refuse depot at The Curragh,
Experts warn that untreated sites can leak toxic leachate or build up landfill gas, posing risks to nearby streams, rivers and ecosystems, and potentially affecting drinking-water quality.
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