The birds in question would generally be cormorants, as shags are a strictly marine species and would be found only near those parts of the canals that are adjacent to the sea.
I understand that Central Fisheries Board monitors and manages fish stocks in the Grand and Royal Canals on behalf of Waterways Ireland, and that this involves the ongoing release of wild fish in areas where numbers are down.
The Central Fisheries Board acknowledges that cormorants can cause minor local problems where new fish stocks have been released into the canals as they are attracted by the increased fish numbers. However, where natural fish populations occur, as is the case along most of the canal systems including the Grand and Royal Canals, it is considered that cormorant predation does not have any significant adverse impact. Cormorants are natural fish predators and the predation of fish by birds and animals is a part of the natural ecological cycle that supports balance and sustainability in the population of different organisms, including fish.