Sligo Borough Council has expressed concern that work being carried out to provide extra spaces at Sligo General Hospital's car park could permanently damage a national monument at Forthill.
Sligo Borough Council has issued a warning to the HSE West about unauthorised work being carried out there.
The site is a protected structure and it is claimed that part of it has been disturbed by construction work to pave way for additional car parking spaces.
Forthill is a walled fort and one of two protected structures in the borough of Sligo, the other being Sligo Abbey.
The fort was the last stronghold held by the Jacobite forces when they were driven into Connacht by William of Orange, following the defeat of James II at the Battle of the Boyne in 1690.
Brian Scanlon of the Green Party said that it is "outrageous" that construction workers were not warned about the Forthill structure.
"It is an absolute disgrace that mounds of earth were just thrown in the air to clear the way for extra car parking spaces.
"This area has never been excavated and there is a complete lack of planning here.
"Why were these men not told of the heritage at the site or an archaeologist brought onsite to show them where and when they could dig?"
He said that Forthill is an important part of Sligo's heritage, but that the area has "slid into disarray".
Sligo Borough Council, in a letter to Sheila Smith, general manager of Sligo General Hospital, said it had come to their attention that unauthorised development was being carried out at land west of the car park at Sligo General Hospital.
"If the following investigation finds that unauthorised development is being carried out, an enforcement notice will issue.
"The planning authority may also apply to the High Court for an injunction to ensure the land is restored to its original condition," it stated.
The letter also states that a person who carried out or is carrying out unauthorised development shall be guilty of an offence on conviction to a fine not exceeding €12,697,381 or imprisonment not exceeding two years.
HSE West has confirmed that a total of 75 additional spaces have been created through the realignment and marking of spaces in Sligo General Hospital's car park.
They say that the work is in response to Donegal Cllr Seán McEniff's call for an extension to the car park at Sligo General Hospital and for all hospitals in the western region.
"It will improve the efficiency of parking, through the development of additional unused areas, said a HSE West spokesman.
"We are continuing to monitor the situation."