Operator Stena Line cancelled four of its ferry sailings between Scotland and Northern Ireland after one crew member tested positive for Covid-19.
Its 19:30 BST departure from Cairnryan and 23:30 BST departure from Belfast were cancelled on Wednesday.
On Thursday, the same incident led to the company calling off its 03:45 BST departure from Cairnryan and its 07:30 BST departure from Belfast Harbour.
"We apologise to our passengers for the inconvenience," Stena said.
The company said it had acted in accordance with its "comprehensive safety protocol procedures".
BBC News has more.