Thousands of pensioners and other welfare recipients face delays in getting payments next week due to a 24-hour strike by postal workers.
The action by the Communications Workers' Union will close down services on Wednesday, but could cause knock-on disruption to payments on Thursday and Friday, the company warned.
Announcing a "national day of protest" on Wednesday, the union said the action was in response to "wanton destruction" of the postal service by the company. As well as a strike on the day by postal workers, the union is to stage a protest march through Dublin, culminating in a "major public demonstration" outside Leinster House.
The decision to stage the action, taken by the union's national executive yesterday, followed a ballot by CWU members in favour of a strike by a nine-to-one majority.
Mr Steve Fitzpatrick, the union's general secretary, said further industrial action was inevitable if An Post continued "to ignore our rights" under various agreements.
"The day of protest next Wednesday is the first step in a vigorous campaign of action by this union and its members to rescue the national postal service from wanton and senseless destruction by an incompetent and untrustworthy management at An Post," he said.
In a statement, the union said the strike was in protest at "clear breaches" by the company of national agreements including Sustaining Progress, the 2003 SDS rescue plan, the July 2000 Transformation Through Partnership deal, and more recent agreements brokered by the Labour Relations Commission.
The list was dismissed as "a fantasy" by a spokesman for An Post. He said many of the issues referred to by the union were covered by the current talks process on the future of collection and delivery arrangements. Talks on those matters recently concluded at the LRC and are likely to be referred to the Labour Court for adjudication early next year.
Regarding the planned closure of the company's parcels division, SDS, the spokesman said that was "a business decision for the board" and was "not an industrial relations matter".
A company statement described the CWU strike plan as "a thoughtless rejection of the legitimate interests of pensioners, the unemployed and others dependent on social welfare payments.
"Thousands of welfare recipients will be unable to claim their benefits during the 24-hour closure and thousands more, due to receive payments on Thursday and Friday, will be similarly affected," it said.
The spokesman explained it would not be possible to move money into post offices on Wednesday for payment the following day.
That difficulty and others would contribute to a knock-on affect which would disrupt services on Thursday in particular.
A fresh warning about the serious financial difficulties facing An Post was issued yesterday by the Minister for Communications, Mr Dempsey.
In an interview with The Irish Times, he said he did not know if people realised the urgency of the situation. "Unless we have a top quality, efficient and customer-friendly service in An Post, it is just going to be wiped out," he said.