The incident occurred at Leixlip bridge on the Dublin/Kildare border during the annual canoe and kayak race on Saturday 26 September.
In the video, British kayakers Sam Weller and Steven Bush can clearly be seen fighting against the fast current after their boat crashes into the bridge and splits in two.
But volunteers from Dublin Scouts immediately raced into action from the bridge overhead, abseiling down to the water to ensure the men's safety.
Bush confirmed that their brush with danger "hasn't put us off kayaking and we can't wait to come back next year and hopefully complete the race."
Neil Fleming and Robin Koenders's K2 was the fastest vessel home at this year's Liffey Descent, the 56th running of the canoe and kayak race, as previously reported on Afloat.ie.