The Cairnryan - Larne sailings were disrupted for several weeks after the company sacked 800 workers last month.
The mass dismissals led to protests and warnings from unions that they could try to blockade Cairnryan and persuade customers to boycott the company.
P&O Ferries revealed the return to normal sailings in a tweet to customers on Sunday morning.
The route is served by two ferries, the European Causeway and the European Highlander. According to the the Marine Traffic ship locator website both were back in operation.
The European Causeway was detained for two weeks over safety concerns following an inspection by the Maritime and Coastguard Agency.
When inspectors examined the European Highlander last week the found a "small number of deficiencies" which had to be put right before it could come back into service.