Thousands in Wicklow Town and surrounding areas have been left without electricity.
The fault at the Ballybeg substation began just after 11.30pm on Thursday night.
Customers in Glenealy, Ashford, Rathnew and Brittas Bay are also affected.
The ESB powercheck app estimated power will be restored at 3.15 am on Friday morning.
While the message below was posted on the ESB Twitter page, lets hope the app is correct.