Around 1170 a Cistercian Abbey was founded on what is now Ashe Quay. There the monks built the town’s first weir somewhere by Ashe Quay/Abbey Street. A settlement grew around the abbey, Sancta Maria de Castro Dei, or Our Lady of the Camp of God. From then the area was known as Mainistir Fhearmui (the Monastery of the Men of the Free Plain).
At the dissolution of the monasteries in 1540, the abbey and lands passed to various English landlords. In 1626, Richard Boyle, First Earl of Cork, ordered a timber bridge for the Blackwater. Two years later it was washed away in “an extraordinary flood”. A 13-arch stone bridge replaced it in 1687, itself replaced in 1865.
If Professor Devoy is correct, then presumably we can blame the earl for building his wooden bridge and causing all the subsequent floods.