The tropical storm is now churning through the Atlantic Ocean around 715 miles (1,145 km) north-west of Portugal's Azores with maximum sustained winds of 45mph, according to the US-based National Hurricane Center.
The rain is expected over a relatively short period of time for many areas across Ireland on Saturday night.
The local authority says the situation will continue to be monitored.
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The forecasters said there will be drier and brighter intervals towards evening, but showery rain will continue to affect parts of Ulster and Connacht. The system should merge with a frontal zone this weekend and bring some wind and rain to the UK.
Tomorrow could see highs of between 20C and 25C in Leinster and Munster and 18C to 20C in the north and west. Breezy as well with fresh and gusty south to southwest winds, strong in some coastal areas.
Fortunately, while Sunday will start off damp with rain and mist, the weather is expected to pick up quickly in the morning for a mostly dry day. Very mild, humid and misty overnight, with drizzle at times in lows of 14 to 15 degrees Celsius.
Sunday night will remain sticky as Ernesto begins to depart, with the tail-end of the storm leaving humid air and wet and windy conditions across the North West.