More than 50,000 households, businesses and commuters were hit with early morning delays and a lack of supply after power was lost to large parts of Dublin.
Passengers were forced to wait in trains stranded between stations for up to 30 minutes after a power outage at an ESB substation resulted in electricity being cut off to parts of the northside.
The ESB said that plant failure at one of the biggest substations in the country caused the fault yesterday.
In all, 26,000 domestic and business customers were affected, and some 30,000 train passengers. Motorists were also delayed as traffic signals failed.
In a statement, the utility said power was restored in just over an hour after the fault was detected.
"ESB Networks can confirm that the fault impacting on 26,000 customers in Dublin was caused by a plant failure at its Finglas 220kV substation, the biggest in the country," it said.
"Our crews immediately mobilised when the fault occurred at 7.20am, safely disconnecting the piece of plant from the network, before commencing remedial works.
"All customers had their supply restored by 8.25am.
"We apologise to customers impacted for the obvious inconvenience caused."
The outage comes just days after 4,600 homes and businesses in Dublin city centre were left without electricity due to a fault with a transformer in Temple Bar, which was likely caused by construction works.
Among the businesses affected in that incident on Monday included Dublin City Council and the National Museum, with traffic signals also hit in Dublin 1 and Dublin 2.
In the latest incident, the ESB said that hospitals were not affected, as they would have switched to back-up generators.
However, there were significant delays to Irish Rail commuter and Dart services.
All Dart services between Clontarf Road and Malahide/Howth, and northern commuter line, were suspended between 7.25am and 7.55am. Four trains were caught between stations, but the remainder were halted at stops.
"It also impacted the morning peak with delays of up to 30 minutes up to 10am to do service recovery," an Irish Rail spokesman said.
Dublin Bus accepted rail tickets on Northside services between 7.20am and 8.30am, and for Southside services between 9am and 10am.