opened on 26 August 1879 and named after Isaac Butt (who died that year), leader of the Home Rule movement.

The swing section, made of wrought iron and weighing 200 tons, ran on a series of cast-spoke wheels and was powered by a steam engine, which was housed on a timber pier on the downstream side of the bridge. The swing action allowed boats to pass and berth in the river as far upstream as Carlisle Bridge (now O'Connell Bridge).