P&O Ferries has reinstated a full service from Scotland to Northern Ireland.
The Cairnryan to Larne route had been interrupted after the firm replaced nearly 800 of its workers with cheaper agency staff.
The sacking sparked anger among unions and political representatives.
The European Highlander ferry, which is one of two ferries which operate on the Irish Sea route, was subjected to an inspection by the Maritime and Coastguard Agency.
On Sunday morning P&O informed customers via Twitter that the services from Larne to Cairnryan have resumed sailing.
Five departures from Larne and four departures from Cairnryan were expected to take place on Sunday.
The European Causeway and the European Highlander ferries were operating on the route on Sunday afternoon according to the Marine Traffic app.
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