Rough sea as sandbags are put along to protect the walls at Sandymount in Dublin from Storm Christine.Sasko Lazarov/Photocall Ireland

Towns and communities across the western seaboard are counting the multi-million dollar cost after Storm Christine finally left Irish shores.

Experts reckon the clean-up operation will cost in the region of $500 million after one of the worst storms in living memory.

The Irish Independent reports that Storm Christine is thought to be the most prolonged and destructive storm experienced in two decades.

The three day storm destroyed coastal defences, washed away roads and damaged homes and commercial buildings.

The report says many coastal communities have been left isolated by extensive floods and without power after winds reached over 75mph in places.

Over 5,000 homes were left without electricity and 11,000 report phone line issues caused by the storm.

Ireland’s weather service Met Eireann has said Storm Christine was the most prolonged storm since the 1990s.

County councils across the country spent the day calculating how much essential clean-up operations will cost.

Damage includes a large sinkhole on a street in the Waterford resort of Tramore, which was caused by the huge waves battering the coast.

The local council has poured four truckloads of concrete into the hole in a bid to stabilise it.

Ireland’s Coastguard service was forced to issue a warning asking people to stay away from piers after a man was swept into the sea at Howth Harbour in Dublin.

PHOTOS - Horrific damage caused by two storms that hammered Ireland's coast

Onlookers also put themselves in danger in Dun Laoghaire as they watched huge waves break.

Coastguard spokesman Declan Geoghegan asked members of the public to keep themselves at a safe distance from the sea.

He said: “We had an incident yesterday evening with a person on the east pier on Howth. Suddenly they were swept off it and suffered a broken ankle.

“Fortunately they were swept onto the lower pier and not out to sea and they suffered injuries.

“It was a man on the pier, obviously he was getting as close as he possibly could to observe the waves breaking and a wave comes in and just sweeps him off the pier wall.

“Then later on we had a further incident in Dun Laoghaire on the East Pier where a number of people were obviously down watching the waves breaking and with the assistance of the coastguard and the gardai (police) we had to close access to them and keep them monitored.

“That’s happening right around the country and unfortunately we can’t monitor all areas.

“But this is becoming quite a pastime for some people and posting pictures on social media but they are putting themselves in extreme danger.”

Geoghegan warned that seas will remain rough as the storm moves on.

He added: “Although it will moderate later on, you still have the tide, winds and high swells which will very quickly sweep someone into the sea before they can get out of the danger area.

“There is another aspect to it, it’s also putting rescue teams at risk for no good reason. Obviously people want to go an observe this and maybe take pictures, but always remain at a safe distance.

“Stay at very, very safe distance from breaking seas and do not go out on exposed areas such as cliffs, piers and harbour walls as people can be swept off before you know it.”

PHOTOS - Horrific damage caused by two storms that hammered Ireland's coast