Translink has said it remains committed to operating the cross-border express bus service to Dublin after the Irish transport authority decided to suspend their service to Belfast.
Bus Eireann's inter-city services are facing the axe due to the impact of Covid-19.
Expressway routes between Dublin and Cork, Dublin and Galway, and Dublin and Limerick are likely to end early next year. The Dublin-to-Belfast service will be suspended indefinitely.
Bus Eireann said in a statement that it plans to expand city and commuter services.
The end date for the inter-city services has not been decided but it is likely to be next year when emergency support for routes expire.
"The X51 service from Galway to Limerick and the X1 Dublin-to-Belfast service have not been in receipt of emergency funding and services will formally cease on these routes over the next few months," the statement said.
Measures are being taken "in response to significant medium-term challenges posed by Covid-19 in order to safeguard Expressway's remaining 14 national routes and meet demand required on other city and commuter services".
The decisions were taken at a board meeting yesterday. The company said it had an opportunity through July stimulus measures to increase services in Cork, Galway and Limerick, Louth and Meath.
Chief executive Stephen Bus Kent said it decided to withdraw from a small number of routes to avoid job losses.
Siptu sector organiser John Murphy said the cuts were "short sighted, flawed and a kick in the teeth for workers and passengers".
A Translink spokesperson said: "Due to the ongoing challenges and impact of Covid-19, Bus Eireann, our current partner on the X1 cross border Belfast to Dublin route, have taken the difficult decision to suspend their service indefinitely following a review and an analysis over the past number of months.
"This move by Bus Eireann will be introduced formally over the coming months.
"Translink remain committed to operating this route and will work with Bus Eireann to manage the transitional period in order to maintain services for passengers along this corridor.
"We will communicate any changes on the timetable and work to minimise the overall impact on our passengers."
Translink said it would work with the Irish authorities "to ensure continuity of services."