The span, verified by Dublin County Council in 1990, measures 110 feet, or 33.5 metres, and yet the whole thing rises only about 6.7 metres at its highest point. That shallow, almost casual curve is the point. The bridge belongs to a category engineers call segmental arches, meaning the arch forms only a segment of a circle rather than a full semicircle, giving it a low rise-to-span ratio of 0.20. The effect from the riverbank is of something almost horizontal, a structure that appears to float rather than leap across the water.

The bridge dates to 1814, and its origins were quietly complicated. It replaced an earlier eighteenth-century crossing located about 160 metres downstream, which had itself replaced a medieval bridge a further 80 metres to the west. No bridge appears at the current location on John Rocque's 1760 map of County Dublin, so the site itself is relatively modern by local standards. The architect, and probably the contractor, was a George Knowles, a double role that Bernard Mullins, speaking to the Institution of Civil Engineers of Ireland in 1859, explained was common at the time due to a shortage of competent contractors and the absence of banks willing to finance such work. The cost came to more than £9,000, a sum Mullins considered excessive given that the foundations presented no difficulty and suitable stone was close at hand. The precise date of construction was established by the researcher de Courcy, who traced the cast-iron balustrades to the Phoenix Iron Works of Dublin and confirmed they were manufactured in 1814. The engineering is worth examining on its own terms: the arch springs from less than three feet above solid bedrock, and the skewback, the angled stone surface from which the arch launches, is formed from three stones set at 45 degrees, an unusual arrangement for a span of this size, braced by pier abutments 1.8 metres thick on each side.

The bridge carries the main road through Lucan village and is straightforward to reach from Dublin city, roughly 13 kilometres to the east. The best view of the arch itself is from the riverbank rather than the road surface, where the see-through iron balusters of the parapet and the shallow spandrels, the triangular spaces between the arch and the roadway above, allow the full geometry to read clearly. There are two other bridges spanning the Liffey in Lucan, so it is worth locating this one specifically; it sits to the east of the village centre. The limestone masonry, coursed and squared with slender ashlar voussoirs, the shaped wedge-stones that form the arch ring, is easiest to appreciate in low winter light when the river is not obscured by summer vegetation along the banks.