Some 1,200 network technicians at the ESB are today due to begin the first major strike at the State electricity company since 1991.
Power cuts are not immediately expected, but the company warned yesterday there could be delays to repair work and new connections.
The strike is to begin at 8am when ATGWU members place pickets on ESB depots throughout the State.
The action is over the extent to which outside contractors are being used to upgrade and modernise the ESB's distribution network.
The ESB says the use of contractors is in line with a comprehensive agreement drawn up with unions in 2001, known as the Pact.
The union claims the terms of the Pact are being breached and, in any event, that agreement has expired.
Both the TEEU and Siptu, which between them represent a further 1,000 ESB network technicians, have instructed members to work normally today.
Their stance has angered ATGWU official Brendan Ogle. He said last week he was disappointed the Irish trade union movement had sunk so low as to invite members to offer "black-leg labour" in defiance of an official trade dispute.
Without referring directly to the dispute, the Taoiseach said last night that divisions between unions could damage the "cause of solidarity" among workers.
Earlier Mr Ahern said: "There's no need for industrial action at all in this case. They should try and take a step back and ease the situation.
"It is entirely unnecessary, unhelpful to the workers themselves, to the organisation they work for and to the general public," he said.
