I have added this description, from Richard Twiss’s A Tour in Ireland in 1775 with a Map, and a View of the Salmon-Leap at Ballyshannon [London 1776] to my second page on the abandoned Main Line of the Grand Canal in Dublin:
The city bason is a reservoir, capable of holding water to supply the city for some weeks, when the springs from whence it is filled are dry; both the springs and the reservoir were dry whilst I was in Dublin. In 1765 a canal was begun to be cut from this place, and intended to be continued to Athlone, which is about seventy English miles off, in order to open a communication with the Shannon; at the rate the work is at present carried on it bids fair for being completed in three or four centuries.
Irish waterways history 