A new Stena Line ferry which temporarily moved from Belfast to Rosslare Harbour in County Wexford has returned.
The Embla first arrived in Belfast on 2 January to serve the Liverpool route.
However, Stena Line switched it to the Rosslare to Cherbourg route, which is in high demand as Irish truckers try to avoid the UK "land bridge" to the EU.
The move was mainly to allow another vessel on that route to undergo maintenance.
Paul Grant, Stena Line's Irish Sea trade director said: "Stena Embla will make one daily return trip between Belfast and Liverpool.
"In March 2020, we launched our new-build Stena Edda onto the Belfast-Liverpool service. Now we will have two ships offering identical services and facilities."
He said the new ship will mean freight capacity on the Liverpool route is up by 20%.
The UK officially finished its formal separation from the EU on 31 December, 2020.
But there has been disruption to trade across the Irish Sea border.
Northern Ireland remains in the EU single market for goods and will continue to enforce EU customs rules at its ports.
Freight shipments on some Northern Ireland routes to Great Britain have been at normal or higher levels in the post-Brexit period as Northern Ireland hauliers try to avoid Dublin port.
Shipping goods from Northern Ireland to Great Britain via Dublin now involves new bureaucracy.
However, hauliers say that while they have full loads going out of Northern Ireland, many trailers are coming back empty, as some firms in Great Britain have reduced their trade into Northern Ireland.
This increases costs for hauliers and may lead to price increases.