… even today:
The swivel bridges at Portumna and Banagher seem to have been badly situated with regard to the passage of vessels, both being placed in the concave bend of the river which makes it difficult, in blowing weather, and going down stream, to shoot the bridge safely.
That is from the Clare Journal, and Ennis Advertiser of 12 September 1850. The swivel bridges of yesterday are the navigation arches of today. The deficiencies of the Shannon Commissioners’ work were evident as soon as they had completed it.
However, although the article from which the sentence was taken is a condemnation of the Shannon Commissioners, the Journal was mistaken in blaming them for Portumna: that bridge was not built by the Shannon Commissioners.
Irish waterways history 