Defence Minister Alan Shatter confirmed that the money was still outstanding under a Memorandum of Understanding.
His department is in negotiations with the UN about the recovery of the money.
In response to a parliamentary question by Fine Gael TD Bernard Durkan, Mr Shatter said the UN reimburses some personnel and contingent- owned equipment costs in respect of contributions of Irish personnel to UN-led operations.
However, he said the only mission for which Ireland was currently entitled to reimbursement of some troop and equipment costs, was the UN Interim Force in Lebanon (UNIFIL).
The total amount owed was approximately €11.5m. This includes €5.1m in respect of the UNIFIL mission and €6.4m in respect of repatriation costs of equipment from the MINURCAT mission in Chad.
Mr Shatter said: “The amount owing in relation to the MINURCAT mission is the subject of ongoing negotiations with the UN.
“The recovery from the UN of the monies owed is pursued with the UN on an ongoing basis by the Department of Defence and the Department of Foreign Affairs and Trade in conjunction with the Permanent Mission of Ireland to the UN in New York.”
Ireland is not entitled to reimbursement in respect of Defence Forces participation in EU-led missions as all troop contributors to such missions are responsible for their own costs.
Mr Durkan said it was normal to have the money paid retrospectively, but said at a time when the country was suffering financially, it was important it be paid promptly.
“The problem would be if we were left awaiting payment for an unreasonable amount of time. One would expect normal procedure for getting the money would be followed particularly at a time when revenue is hard won and hard got.
“I hope the UN keeps up to speed in terms of the payment for the various deployment,” he said.