There is no evidence for bridges in prehistoric Ireland. Fords were used as crossings and the earliest structures appear to have been interrupted causeways, built of stones and punctuated by gaps enabling the water to flow through. An example survives at Skeagh on the river Shannon between counties Leitrim and Roscommon, and it is probable that the causeways (tochair) built at Athleague, Athlone, and Dublin in 1001 by Mael-Sechnaill II were of this form.
The earliest documentary evidence for bridge building occurs in Cogitosus’s Life of Brigit (c. 650), which makes it clear that it was a prerogative of kings and the responsibility of the local community. The oldest known bridge, dendrochronologically dated to 804, spanned the river Shannon at Clonmacnoise. It was over 500 feet long, 10-12 feet across, and consisted of two parallel rows of oak trunks, set 16-20 feet apart, hammered into individual base plates of beams and planks, and driven to a depth of ten feet into the river clays. The form of the superstructure is unknown, but planks, post-and-wattle, and poles are mentioned in the early sources. A somewhat later timber plank bridge, probably of thirteenth-century date, is known from the river Cashen, County Kerry. It was carried on trestles fitted into sole plates that had been pegged into the riverbed. It had a span of 600 feet. The earliest stone bridges were of clapper form. A probable pre-twelfth-century example on the river Camoge at Knockainey, County Limerick, survived until the 1930s.
In the eleventh century, major bridges were built across the Shannon at Athlone, Athleague, and Killa-loe. These were constructed not just to facilitate trade and communication, but also to permit the rapid deployment of armies. At least four successive bridges were built at Athlone in the course of the twelfth century. All follow the same pattern, built by the kings of Connacht to give them easy access into midland Ireland, and destroyed as quickly as possible by the kings of Mide to prevent such incursions. In Anglo-Norman Ireland the responsibility for maintaining bridges rested with the local community, which could rarely afford the costs involved. Accordingly, from the early thirteenth century, "pontage" grants were given to communities permitting them to levy tolls on commodities brought into the town for sale. The monies so collected were spent on building and maintaining the bridge. In the fourteenth century important bridges were built at Kilcullen, County Kildare (1319) and Leighlinbridge, County Carlow (1320). These had the effect of shifting settlements from the older ecclesiastical sites down to the bridging points, where they have remained to this day. Surviving medieval bridges, such as Adare, Askeaton, Slane, Trim, and Babes Bridge, County Meath, are characterized by pointed segmental arches, a width of between six and nineteen feet, arch spans of about twenty feet, prominent cutwaters, and high parapets that were sometimes battlemented. Ancillary structures such as gatehouses, water gates and slips were common in towns while chapels (Dublin) and houses (Baal’s Bridge, Limerick, and Irishtown Bridge, Kilkenny) were also constructed on bridges.
