Six people who were being questioned by gardaí in Co Galway following a surveillance operation under Operation Anvil at the Tynagh power plant were released last night.
A total of 11 people were arrested during the early hours of yesterday morning and four vehicles were seized containing an estimated €50,000 worth of copper wiring.
The recently-completed gas-fired 400 megawatt power plant in Co Galway is run by the Tynagh Consortium.
It is understood that the group had been under surveillance since Thursday.
The four vehicles - three vans and a four-wheel drive - were spotted leaving the construction site at about 4am yesterday.
They were stopped using a Garda stinger device which punctures tyres on impact.
The vehicles, which were heavily-laden, were searched and five people arrested and taken for questioning.
A sixth person was detained in the village of Killimor at about 6.30am.
In the following five hours, gardaí from Millstreet station in Galway were joined by colleagues from Garda stations across the county and ones in Tipperary in a further search along the south GalwayTipperary border.
In radio broadcasts, motorists were asked not to give lifts to hitch-hikers.
Due to foggy conditions, the Garda helicopter could not be used to assist in aerial surveillance.
At about 9am, another five people were arrested in fields in the general south Galway area, according to a Garda spokesman.
All 11 people have addresses in Dublin and one is a juvenile, the spokesman confirmed.
The 11 were taken to Garda stations in Gort, Loughrea, Ballinasloe and Salthill and were held yesterday under section 4 of the Criminal Justice Act.
The juvenile will be subject to the juvenile liaison scheme, the spokesman said. There have been a number of reports of thefts of copper wiring from various power stations, telecom installations and construction sites around the State, according to gardaí, who say that the wiring is melted down and sold for various purposes.
A similar surveillance operation took place in Ballyhaunis, Co Mayo, earlier this week and two men were arrested.
Operation Anvil was established in May 2005, in response to a spate of gangland attacks, primarily in Dublin, and received initial funding of €6.5 million from the Department of Justice to allow the Garda Síochána to work some 15,000 hours of extra overtime per week.
Crime figures released by the Garda Síochána in March showed that the initiative was proving to be "very successful", Minister for Justice Michael McDowell said, with 23 arrests for murder and over 200 arrests for serious assaults since last September.
Property valued at about €5.7 million was also seized in a 10-month period and over 700 searches were carried out by gardaí for illegal weapons.