This is a point that I do not recall seeing before. It arises in a short report from the Freeman’s Journal of 17 July 1876.
SAVED FROM DROWNING. — On Saturday evening a man named Patrick Fitzsimons, while employed with others in getting a canal boat through the lock of the Portobello-bridge, fell into the basin and sank. He rose to the surface in about a minute, and was apparently exhausted, for, after a vain attempt to hold on by the projecting ledge of the boat, he went down again. There now seemed to be great danger of the man’s life being lost, but some of his companions held out one of their long “sweep” oars towards the place where he sank, and when he came up the third time he succeeded in grasping the oar and holding on till he was taken out of the water. He was then in a very weak state, and it appeared very plainly that when he fell into the basin he was not in the best condition to protect himself from accident.
I suspect that the last phrase means that he was drunk. But what is more interesting, at least to me, is that a canal boat was equipped with oars. I do not recall having read that anywhere. But we know little about the design, equipment and operation of nineteenth century canal boats. Oars would certainly be useful for moving around basins and on rivers like the Liffey, but how were the oars pivoted and how many men did it take to row a loaded canal boat?
Irish waterways history 
When I read ‘Sweep oar’ I immediately thought of the type of oar used in Venetian gondolas. Mention is made of a’ canal boat ‘ which I imagine may not necessarily mean a barge so possibly a smaller boat that might be propelled by such an oar ?
No: “canal boat” was the standard term for “barge” on the Grand, and is widely used in press reports of the time.
A sweep is a large, usually long and heavy, oar. Because of its size, a single sweep is often operated by a standing boatman (like the gondolier), who may also walk with the sweep; a rower in a smaller vessel might use two oars. It’s not surprising that a large vessel could be moved under sweeps. Boats on the tidal Suir used 35′ sweeps. You can see some pics of large Thames lighters here.
But I hadn’t realised that Grand Canal boats carried sweeps. Yet it would explain how canal boats could move about the docks on the Liffey in Dublin without having to pay for expensive tows from steam tugs. A third possibility is that pulling boats (oar-propelled tugs) were used; I have no information on that.
bjg