Lock And Bridge, Milltown Road, Ballynavally, Castlereagh, Belfast

Lock 3 on the Lagan Navigation is a single-chamber masonry lock built between 1756 and 1763 under the direction of Thomas Omer for the Corporation for Promoting and Carrying on an Inland Navigation in Ireland. It is one of twelve locks on the Lagan Navigation between Stranmillis and Union Bridge in Lisburn.

The lock is aligned east-west and situated at the eastern end of a cut through a curve of the river to bypass rapids. It measures 21.8 metres long by 4.7 metres wide (71 feet 6 inches by 15 feet 4 inches). The chamber walls are lined with masonry: the lower section consists of quarried rubble blackstone brought to courses, while above this are larger squared sandstone blocks with an admixture of blackstone rubble. Prior to recent restoration, much of the sandstone had eroded and been replaced with brick. During restoration, most of this brickwork was replaced with random sandstone blocks of paler colour than the original, laid to regular courses. The majority of the north side of the chamber is now new sandstone with little brick repair work remaining. The tops of the walls are coped with large dressed sandstone blocks. The original timber floor and original stone cill at the upper (western) end have been replaced with reinforced concrete during recent restoration. Halfway along the north side of the chamber, a vertical galvanised steel escape ladder has been let into the wall (not an original feature). A pair of replica timber lock gates stands at each end, each with its own counterweight beam and paddle sluice gate operated by a rack-and-pinion mechanism. Across the west face of each gate is a timber footway cantilevered out from the gate on galvanised steel brackets. A simple timber footbridge erected in the 1990s spans the chamber between the gates to facilitate access.

Immediately downstream from the lower gate is a small single arch masonry bridge by which horses towing barges crossed from one side of the navigation cut to the other. The arch springs from the sides of the chamber and has a segmental profile with roughly dressed sandstone voussoirs. Its deck is strongly humped and the parapets are coped with concrete.

Approximately 3 metres downstream from the footbridge is a curved lagged mains water pipe running between concrete abutments on either bank. A second water pipe formerly spanned the chamber at a much lower level between the lower lock gate and footbridge but was removed during recent restoration and replaced with a pipe under the lock. At the time of survey, the navigation channel was blocked by a temporary earthen causeway which facilitated contractor access during lock house restoration. A tarmaced public footpath runs along each side of the lock. The former lock-keeper's house stands immediately to the north.

The Lagan Navigation was built between 1756 and 1763 and subsequently extended as a canal to Lough Neagh, opening in 1794. It operated until 1954, when the upper canal section was closed. The Belfast-Lisburn section was not abandoned until 1956. The lock appears on all editions of Ordnance Survey maps from 1834 onwards. The lock was designated a Scheduled Monument in 2003 under Article 3 of the Historic Monuments & Archaeological Object (Northern Ireland) Order 1995. It was recently restored to operational order by Castlereagh Borough Council under the auspices of Lagan Valley Regional Park, with funding from the Heritage Lottery Fund. The lock has group value with the adjacent lock-keeper's house.