Peter Hebblethwaite appeared before the Scottish Parliament’s Transport Committee on Tuesday and was grilled by Jackie Dunbar MSP on issues specifically related to Cairnryan.
However, while Mr Hebblethwaite disclosed that 39 seafarers have lost their job at the port, he was unable to explain why these staff were sacked.
He previously admitted the company broke the law when it chose not to consult with unions over its decision to make redundancies.
Stena Line has stepped in to cover P&O services that have been suspended, with officials in Northern Ireland reaching an agreement with the EU that livestock can go via the Stena route to Belfast and then on to Larne by road.
P&O's European Causeway remains detained at Larne after Marine and Coastguard Agency concerns over crew familiarisation and documentation issues.
European Highlander sailed without passengers or cargo to Larne for MCA checks.
South Scotland list MSP Emma Harper last week attended a protest at Cairnryan where she met people directly impacted and to the assembled crowd and trade union members.
She said: “I repeat the Scottish Government’s position that the manner in which decisions had been taken by P&O, including the method used to communicate the redundancies to staff, was completely despicable.
"P&O must now reverse this decision, properly communicate with staff and their unions and act within the law.”
This sentiment was echoed by Galloway and West Dumfries MSP Finlay Carson who has received confirmation from UK Maritime Minister Robert Courts that the Department for Transport is continuing to work with Scottish officials to ensure alternative arrangements are in place to support freight and passenger traffic.
Mr Carson said: “The minister shared his anger and disappointment regarding the decision taken by P&O Ferries.
"Importantly he highlighted the lack of engagement, of prior notice or empathy for their employees.
"But he took time out to praise Stena Line who put on additional ferries to operate from Cairnryan to Larne.
“What the company has done is completely unacceptable. Seafarers make a huge contribution to this country and many of those affected had dedicated years of service to P&O.
“I was frankly staggered at the way they were dismissed with zero respect.”
The UK Government gave P&O “one final opportunity” to reemploy sacked staff on their previous salaries.
Ministers plan to make it illegal for ferry firms operating from UK ports to pay less than the minimum wage, and efforts to change the law began on Wednesday.
Meanwhile, Dumfries and Galloway Council is meeting today (Thursday), to debate an emergency motion which would exclude P&O Ferries and its parent company DP World from its bid for Green Freeport status for Cairnryan.