Two new ships are to be introduced on to Stena Line's Belfast to Liverpool service in 2020/21.
The first of three new RoPax ferries currently under construction in China is planned to enter service on the company's Dublin-Holyhead route in early 2020.
The two liners on the Belfast route are expected to be introduced in the spring of 2020 and early 2021.
Stena Line has ordered the construction of six new vessels at the AVIC Shipyard in China - with three sailing its Irish Sea routes.
The other three will be chartered out to external parties by sister company Stena Roro.
CEO Niclas Martensson said that Stena Line is investing almost £200m.