Glendun Viaduct, Irragh/Clegnagh Tls, Glendun, Co.Antrim

Glendun Viaduct is one of the finest mid-19th century masonry arch bridges in the Province, representing the apogee of mid-19th century bridge design. Completed in 1839, it was designed by Charles Lanyon, the County Antrim Surveyor, and constructed by William Bald as part of the Antrim Coast Road scheme.

The bridge is a triple-arched masonry road bridge constructed entirely in sandstone, spanning the Dun River on the Cushendall to Ballycastle road. All abutments and piers are formed of sandstone blocks brought to courses, with plain quoins. Shallow buttresses of randomly-sized blocks are attached to the up- and downstream faces of the abutments and piers, tapering inwards on both their faces and edges as they rise. These buttresses are embellished with stepped, vee-jointed sandstone quoins with rusticated and margined faces.

All arches are of semicircular profile with voussoirs detailed as the buttress quoins. The middle arch is the tallest and widest, spanning the actual river, while the flanking arches are of identical size—the northern one spanning a minor road along the left bank of Glendun and the southern one dry in normal circumstances. Since no piers are footed in the river, no cutwaters were necessary. The arch soffits are of coursed sandstone blocks, with a row of projecting stones below arch spring level where the falsework was set during construction. Moulded sandstone string courses run through each arch at spring level and continue around the abutment and pier buttresses. The spandrels are of randomly-sized blocks brought to courses, and a moulded string course runs across each face at the base of the parapets. The parapets are of regularly-coursed rusticated and margined sandstone blocks with similarly detailed oversailing copings having sloping tops. The parapets terminate in out-projecting piers continuing upward from the abutment buttresses, with identical out-projections along the outside faces of the parapets at the top of each pier buttress. The approach walls are of random rubble, and a substantial earthen embankment carries the road across to the southern end of the bridge.

The bridge's original fabric survives intact, though the plaque has been replaced. Its slender proportions and tapering buttresses, set deep within the Dun Valley, accentuate its imposing scale. The quality of its construction, exemplified by its dressed sandstone blockwork and embellished dressings, demonstrates the refinement of mid-19th century bridge engineering.

The Antrim Coast Road, constructed between 1832 and 1842 by the Commissioners of Public Works in Ireland, was the largest civil engineering project in Ireland of its day. The new road on which this bridge sits superseded a previous steeper route via Clady Bridge, a short distance downstream, significantly facilitating social and economic connectivity throughout the area. An 1832 Ordnance Survey map shows a dotted line marking the road as "new road in progress 1833", and the bridge's datestone confirms its completion in 1839. From 1857 onwards, the structure is explicitly captioned as "Glendun Bridge" on all editions of the Ordnance Survey maps.

The bridge has group value with neighbouring structures along this line of road: Craigacat Bridge, Corratavey Bridge, Altheela Bridge, Bush Burn Bridge, and Altadreen Bridge. It is the largest and most visually impressive of all bridges on the Coast Road and is of national significance.

The bridge and road span the deep valley carved out by the Dun River and are surrounded on all sides by fields and patches of woodland.