Ms Sinnott, who visited the 147-acre Gortmore Tailings Pond in Silvermines, Co Tipperary, at the weekend, said it was an environmental blackspot which had been ignored by the Government.
Parts of the grass vegetation on the solidified surface has died, leaving locals exposed to toxic dust laden with cadmium, lead and other toxic materials.
Ms Sinnott said: “The way the grass is dying back, it’s almost like desertification. Some parts were just black, like there had been a bonfire.”
The dust blowing from the tailings pond onto surrounding farmland exceeded acceptable levels over recent months, according to monitoring from North Tipperary County Council.
An EPA report in 1999 described the tailings pond as a continuous threat to animal and human health.
Ms Sinnott, who recently became a member of the European Parliament’s Environment Committee, said she would raise the issue with the European Environment Commissioner.
The tailings pond was created by mining company Mogul Ireland, which piped liquid mining waste there until 1982. It has an obligation under its licence to rectify the lands affected.
However, the company’s recent plan to spread organic waste on the dump was rejected by locals, the Environmental Protection Agency and the Department of Communications, Natural Resources and the Marine.
Ms Sinnott said the Government should pay for the rehabilitation of the site and pursue Mogul for the costs.
“The local people need to go after the Government, who were supposed to protect them. It is much harder to target a private company,” the South MEP said.
The Gortmore Environmental Action Group said it wanted a layer of impermeable material placed on the surface, followed by a covering of topsoil.
But Mogul Ireland is proposing a revamped version of its earlier organic waste plan.
The Department of Communications, Natural Resources and the Marine said negotiations were progressing “very positively” and would be completed within months.
Principal officer Michael Daly said: “It’s up to the local community to take stock then. We will be consulting with them.”
He added that the Government had no legislative power to rehabilitate the site on its own.