One woman died when a tree toppled onto her car, while a man was killed in an accident while trying to clear a fallen tree with a chainsaw. A second man was also killed by a tree falling onto his vehicle.
The third death occurred at Ravensdale, north of Dundalk in the northeast, a few kilometres from the border with Northern Ireland.
Police said they were at the scene of a "serious traffic collision".
"The alarm was raised at approximately 2:45pm (local time) when a car was struck by a tree. It is understood a male occupant of the car has been fatally injured.
Ophelia, the largest hurricane ever recorded so far east in the Atlantic Ocean and the furthest north since 1939, was downgraded to a storm before it hit the Irish coast but nonetheless wrought havoc, leaving 360,000 customers without power.
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