Greystones South Beach and Dun Laoghaire harbour approx 20 km - which one is start and end depends on wind direction. This has been a course for the Ireland Coast Paddling Championships
Fountainstown Beach - Ringaskiddy (Cork Harbour Run) 8 km Downwindable.com
Dunbrattin HeadPier to Tramore Pier 13 km “works in Westerly, North westerly. Gets rough coming up to and past Metalman but great runs too. Cliffs alongside entirety of course so plenty of clapotis. Best but not neccessary at low tide when more of the wave energy is retained by beaches/ cliff base. Usually a significant SW swell coming from 4 o’clock direction (Ed. ? assume this means 4 o'clock as if 12 is straight east since this is a run to the east). Nothing surfable in S or SW. Can easily be extended up to 17kms starting at Bunmahon beach or 30kms starting at Stradbally or 40 kms starting at Ring Peninsula Harbour” - from irelandsurfski.com, video, Downwindfun