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.

Staff Say Unsafe Pressure At Naas Hospital Is Worsened By Uncovered Sick Leave And Vacant Positions
Inside The Celbridge Lab Where Evidence Helped Identify 32 Missing People Last Year
Man Assaulted By Two Males In Newbridge After Refusing To Hand Over His Jacket
Taoiseach Says Patients Shouldn’t Pay for Blister Packs - But Says State Can't Pay For Everything
Council Executive Asked Naas Cllrs To Refuse Free Xmas Parking To Encourage Use Of "Sustainable" Travel
Gunman Steals Cash From Centra In Naas Before Being Tracked Down By Gardai
Woman Arrested At Dublin Airport Detained In Kildare Garda Station Over Alleged Series Of Thefts
Vulnerable People And Those With Disabilities In Kildare Left Waiting A Year For Basic Advocacy Support