Workers are protesting against cost-cutting proposals
Bus Éireann services will come to an indefinite standstill from Monday March 6th.
SIPTU members say the strike at the company is "one which must be won" to secure a future for all semi-state workers and rural Ireland.
Workers are protesting against proposals for "immediate cost savings" to be implemented from the same date.
The board of company approved the measures earlier this week, which they say is due to the "perilous state" of the company’s finances.
In a statement Tuesday, the firm said: "These measures are vital to ensure that the company remains solvent, and can continue to trade as a going concern.
"The company must deal with its challenges directly, and the board have a duty to ensure Bus Éireann is financially sustainable and therefore must take the necessary steps to secure this.
"Further dialogue aimed at urgently resolving these challenges would be welcome.”
While a letter from SIPTU division organiser Greg Ennis to all SIPTU members in Bus Éireann, dated March 3rd, states: "The attempt, by the recently appointed senior management of Bus Éireann and their specialist external advisers, to savage our members moderate terms and conditions and further privatise a proud state owned transport company must be resisted at all costs, both for the good of the rural travelling public who deserve and depend on the essential Bus Éireann service and our members.
"This country and the travelling rural public deserve and require a properly funded public bus service and the Government must fund it appropriately and not simply continue to manufacture the privatisation of Bus Éireann by stealth in accordance with its unworkable policies".
The letter also pledges "unflinching support and solidarity" of the SIPTU TEAC Divisional Committee - and its 25,000-membership across its transport, energy, aviation and construction sectors.
This will again raise concern that solidarity strikes could be on the cards form workers in Dublin Bus and Irish Rail.
Ennis adds: "This is a dispute that can be won and must be won for the sake of not just our Bus Éireann members and their families, but also for the thousands of SIPTU members right across the commercial semi-state sector and indeed the thirty-two counties of Ireland, who no doubt will support your struggle at every opportunity.
"We must do all that we possibly can to protect some semblance of dignity at work and justice in Irish society."