A crushed van in Limerick
It is the biggest power outage in the country for more than 15 years, with more than 2,000 separate faults across the network.
Among the worst affected areas are Cork, Kerry and Clare.
The worst of the storm battering the country is expected to pass by 7pm, but not before bringing winds which could reach 100mph (160kph).
A Status Red warning for Leinster, Munster and Connacht has been issued by Met Eireann, with dangerous winds, heavy showers and thunderstorms anticipated.
A major emergency has been declared in Kilkenny, while a tornado was reported in Roscommon.
Power supplier ESB Networks has warned some 200,000 customers could be without power overnight as the dangerous conditions hamper repair work.
"The electricity network has sustained extensive damage as a result of today's storms, and we are dealing with a situation that is as bad if not worse than anything that we have seen in the past decade," Managing Director Jerry O'Sullivan said.
Road, rail and air travel across the country have been significantly affected.
Drivers are being urged to look out for excess surface water and risk of floods in places. Motorists are being urged to check local weather and traffic reports before travelling.
There are reports of downed trees and telegraph poles in parts of Cork, Kerry, Tipperary and Clare. AA Roadwatch received reports of flying debris on the Dingle peninsula, including shed roofs.
At Cork airport, a number of flights have been diverted or cancelled. Shannon airport has been forced to close due to the extreme weather.
Irish Rail customers on all routes in Cork and Kerry should expect disruption and significant delays due to fallen trees and debris.
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