The agency that manages the Republic's national electricity transmission system is investigating a blackout that occurred in some parts of the country last weekend.

Eirgrid, which manages the National Grid, the network through which electricity is transmitted from power stations' sub-stations, says the blackout occurred on Friday, July 13th and affected less than 1 per cent of the market. A spokesman said it happened when one of the generating stations in Dublin Bay unexpectedly cut out and another plant that was on reserve failed to come on line quickly enough.

"It was a minor incident, but we are investigating it," the spokesman said.

Launching the agency's annual report last week, Eirgrid chief executive Dermot Byrne said that power station availability in the Republic was unsatisfactory.

The report states that 78 per cent of power stations were available to generate electricity in the Republic during 2006.

Mr Byrne said that Eirgrid wants to see 80 per cent availability and pointed out that industry best practice is 88 per cent. Power stations have to close regularly for maintenance and other reasons.

The Republic's power plants have the capacity to generate over 5,800 mega watts (mw) of electricity, enough to power close to six million homes.

Power demand hit a record peak of over 5,035 mw last December, and while demand is expected to grow in the Republic, it should slow from recent 4-5 per cent increases to around 3 per cent annually.

Eirgrid believes that, taking into account three planned new power stations with capacity of 1,200 mw and ESB proposals to close four generators with 1,300 mw capacity, electricity supply is secure until 2011.

In the long term, the industry estimates that at current rates of growth, the Republic will need to build a new power plant every two years.