Senior officials from the Department of the Environment, including its secretary general, Mr Niall Callan, have been interviewed about the circumstances surrounding the appointment of Ms Monica Leech as a public relations consultant to the Department in 2002.
Mr Callan and other senior officials have held face-to-face meetings with Mr Dermot Quigley, the former chairman of the Revenue Commissioners.
Mr Quigley is due to report to the Taoiseach next week, although it was unclear last night whether he had interviewed the Minister for Transport, Mr Cullen, who was Minister for the Environment at the time of the appointment.
A spokesman for Mr Cullen said the minister would not be commenting on any aspect of the inquiry, pending its outcome.
Ms Leech is a close political supporter of Mr Cullen, although he has always denied any impropriety in the appointment, while the Department of the Environment has maintained that standard tendering procedures were followed in awarding the contract.
It is understood that the interviews with environment officials focused on the tendering procedures used to appoint Ms Leech, from Waterford, to two PR contracts with the Department, in 2002 worth over €300,000.
It has also emerged that Mr Cullen will be interviewed next month by the Public Accounts Committee on the role of Ms Leech in the electronic voting controversy. Ms Leech was involved in the public relations strategy and sat on the committee which awarded a €4 million public relations contract.