Thrill-seekers planning to jet-ski on the River Liffey have been told not to by the Dublin Port Authority.
The move follows sightings of high-speed skiers during the St Patrick's Day celebraitons.
But their activities were not part of the official celebrations, and the Dublin Port Company has now warned that the sport is against the law.
A spokesman said: "The use of jet-skis on the Liffey is forbidden under the Irish Harbours Act (1996). Penalties include fines of more than £1,200 and up to a year in prison."
The company also warned that the Garda’s water unit regularly patrolled the river to ensure that regulations were not infringed.
Efforts have already been made this year to tighten up on the use of high-speed craft on the Liffey.
The owners of jet-skis and power-propelled small boats have been told that they are liable to prosecution if they do not stick to the rules.
An official notice declared: "Water-skiing or aquaplaning behind speedboats or fast jet-skiing is entirely prohibited when within 200 metres of any public beach, bathing place or residential property within the jurisdiction of the Dublin Port Company."