Bus Éireann services from Dublin to other cities face the axe due to the impact of Covid-19.
Expressway inter-city routes between Dublin and Cork, Dublin and Galway and Dublin and Limerick will end, probably early next year.
The Dublin to Belfast service will be suspended indefinitely.
Bus Éireann said in a statement it is consolidating, but plans to expand city and commuter services.
The end date for services has not been decided, but is likely to be early next year when emergency supports for routes expire.
"The X51 service from Galway to Limerick and the X1 service Dublin to Belfast have not been in receipt of emergency funding and services will formally cease on these routes over the next few months," said the statement.
It said the measures were 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 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".
He called on Transport Minister Eamon Ryan to intervene immediately and blamed the National Transport Authority for flooding the market with operating licenses.
The National Bus and Railworkers Union called on the government to step in and help retain services.
"The politicians, particularly those that represent the constituencies which will be impacted by this decision to cull routes, should ponder on the fact that Bus Éireann, and Bus Éireann alone, in the vast majority of areas across Ireland, was the only bus service left standing during the Covid-19 induced lockdown," said general secretary Dermot O'Leary.
"Without such services, large parts of Ireland would have been cut off, essential workers would not have had public transport availability to attend vital workplaces, such as hospitals, doctors surgeries, pharmacies and retail."