1995:239 - CLONMACNOISE, Offaly

Since June 1994 divers, under licence from the OPW, have been searching the River Shannon at Clonmacnoise for evidence of a bridge. The Annals of Clonmacnoise refer to a bridge at the site in AD 1158. In the spring of 1995 a number of timbers were located on the river bed. The OPW subsequently contracted IUART to carry out a pre-disturbance survey of the river bed in the vicinity of the timbers in order to determine their function.

A survey grid was laid on the river bed, covering an area of 10,000m2. Survey work was made extremely difficult by the almost complete lack of underwater visibility. However, a bridge structure was found to span the river on a north-south axis directly below the castle site. On the northern bank of the river a row of vertical posts were found leading across it. In all, seven pairs of posts were located, running for a distance of 42m. South of this row a series of interesting structural features began to show in line with the vertical row. This consisted of an upright post with a longitudinal timber cutting through a half-lap joint on the vertical. This longitudinal timber would appear to constitute the side supports for the bridge superstructure. Two transverse timbers cut this longitudinal one at either side of the vertical, possibly acting as cross-member supports for the walkway. This structural feature is visible in four locations in the centre of the river. An initial theory suggests that this is the central part of the original bridge which has either sunk or collapsed onto the river bed. The existing remains on the river bed show that the bridge had a span of at least 116m. Dendrochronology samples from the double post row on the northern river bank produced a date of AD 804, which was supported by radiocarbon dating. This shows that the bridge was constructed in the early medieval period and is not related to the castle site as originally thought. This date makes the Clonmacnoise bridge the earliest known bridge site in Ireland.

Six dugout canoes were also located during the course of the survey, lying close to the bridge. One of the dugouts contained an axehead as well as a whetstone. The concentration of so many canoes in such a location would suggest that this area has a long tradition as a crossing or ferry point.

Unfortunately the site is under threat from a number of sources, including the large amount of boat traffic in this area, as well as from unauthorised diving and metal-detecting on the site. It is intended to carry out further work on the site.