Two men have pleaded guilty to the illegal disposal of waste, which included chemical waste and human blood as well as "risk waste" that could transform into gas, as a result of a Garda investigation.
The investigation was prompted by a Wicklow County Council complaint. Dublin Circuit Criminal Court heard it could cost up to €10 million to make the Co Wicklow dump safe.
John O'Reilly (64), Robertstown, Naas, who then owned the site at Whitestown, Baltinglass, pleaded guilty to disposing of waste that could cause pollution on a date between November 2000 and December 2001.
Neville Watson (41), Kilmurray, Trim, Co Meath also pleaded guilty to not having a licence under the Waste Management Act for use of the site for dumping waste on the same dates.
Both men, along with others, are also facing civil proceedings in the High Court initiated by the council. A fine of up to €10 million could be imposed in the case, the court heard.
Gardaí became involved in 2001 after the complaint by the council, which the court heard had itself also used the 20-acre site informally for dumping waste for some years.
O'Reilly had planning permission since the 1980s to operate the site as a quarry to extract sand and gravel and did this mainly by licensing others.
He told gardaí that some of these users unloaded waste on the site before taking sand or gravel. He made an arrangement with A1 waste in 1998 that waste could be deposited for £20 (€26) per load.
O'Reilly was then approached in 2000 by Watson, on behalf of Swalcliffe Ltd, trading as Dublin Waste, who agreed to pay £62.50 (€80.25) per lorry load to deposit waste there.
Watson had his own waste disposal trucks but worked for Dublin Waste and another man who would be before the court on another date in relation to illegal waste dumping.
The court heard it was originally agricultural land and one of the conditions of the quarry planning permission was that the land would be restored.
Dónal Ó Laoire, an environmental consultant, said the site had been very well covered and concealed and it took some time to detect the illegally dumped waste- which included chemical waste and human blood.
He was concerned about "risk waste" that would transform into gas or contain contaminants which could escape into the Carrigower river, which had the highest quality river water and flowed into the River Slaney.
Drainage ditches from the site were also going directly into the Carrigower river.
Mr Ó Laoire said that when a bore hole was dug on the illegal site, Sonia Deane, assistant engineer with Wicklow County Council, collapsed after being exposed to gas and was taken to hospital.
Gas tended to come up from these sites and could cause explosions when it entered residences.
Mr Ó Laoire said the council was "probably looking at a cost from €6 million to €10 million", depending on factors, to deal with the waste.