Mick Farrell of the HBA has pointed out that, on the Historic 6″ Ordnance Survey map (~1840), the first lock on the Royal Canal is downstream of Newcomen Bridge whereas on the Historic 25″ (~1900) the lock is upstream of the bridge.
Industrial Heritage Ireland has created a page giving the history of the railway crossing at Newcomen Bridge. However, it would be nice to have some documentary evidence about the resiting of the lock — and about the headroom under the bridge before the lock was moved.
Irish waterways history 
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