Warrenpoint Port has said it handled a record high of 3.6 million tonnes of cargo in 2018 with shipments including wood chip, cement, animal feed and steel.
And it's aiming for further growth following Brexit after adding capacity on the Warrenpoint-Heysham route.
And the tonnage makes the Co Down port the second largest in Northern Ireland - ahead of Foyle Port, which had gross tonnage of 2.166 million tonnes in the year to March 2018.
More than 126,000 units passed through the port, with an increase in freight units of nearly 5%.
It has also spent £3m on buying a new crane and refurbishing others.
Belfast Harbour is the province's biggest port, and last week said its cargo had surpassed 24 million tonnes for the first time.
Warrenpoint Port said the value of goods had also hit a record level of £6.2bn. It now handles 12% of Northern Ireland's seaborne trade.
Chief executive Clare Guinness said: "2018 was another record year for Warrenpoint Port following four successive years of growth.
"Much of the increase can be attributed to the success of our core roll-on roll-off freight service while we have also diversified the range of bulk goods that we handle as we look towards further growth in 2019 and beyond."
Warrenpoint Port trades with countries including Spain, Italy, Sweden, Belgium, Germany, Ukraine and the Americas.
The increase in freight units followed the deployment by Seatruck Ferries of two bigger freight vessels for the Warrenpoint to Heysham route, following growing demand for unaccompanied trailer space.
Seatruck has said the vessels will allow it to carry 30,000 extra trailers every year, allowing for growth following Brexit.
Ms Guinness added: "As we look towards further growth in 2019 and beyond, we have plans to continue our investment programme over the coming months as we aim to improve port infrastructure, plant and equipment.
"This will allow us to be more flexible, more efficient and open up more space, enabling trade to grow in line with expectations and widen the scope of goods we can handle.
"Last year saw the commencement of wood chip exports from Warrenpoint for the first time while we also handled our largest consignment of wind turbines."
The port now employs 70 people directly, while 200 work at the facility.
Belfast Harbour's growth to over 24 million tonnes of cargo was led by a strong increase in freight vehicles being carried on Stena Line's services to Great Britain, which rose by 3% to a record 532,000 freight vehicles.
Co Tyrone fuel company LCC also increased its imports and exports of industrial coal by 37%.
Figures for Foyle Port for the year ending March 2018 give a gross tonnage of 2.166 million tonnes, with imports making up 1.722 million.
However, exports rose from 83,913 to 102,449.