Two on the whole canal
The skew bridge
The Royal Canal has 146 kilometres of waterway and only two skew bridges — bridges built on a slant because the road they carry crosses the canal at something other than a right angle. Ballinea Bridge is one of them. The mason's trick is to lay each course of stone on a helical line so that the arch can twist while still standing up; it is the kind of problem that took canal-age engineering to solve and modern road-builders to forget. The original 1810s structure is still in use. Walk down to the harbour, look up at the underside of the arch, and you can see the courses tilt.
Why there is no lock here
The summit pound
The Royal Canal climbs out of Dublin through twenty-five locks, reaches its summit at the western edge of Mullingar, and then runs level — the summit pound — for about eleven kilometres before the 26th lock at Coolnahay starts the long fall down to the Shannon. Ballinea is on that pound. The summit was reached in 1806; Coolnahay and the descent followed three years later when the lock cuttings were finished and the canal could finally push west into Longford. For a boatman heading from Dublin to the Shannon, Ballinea is the point at which the work of climbing is over and the work of dropping has not yet begun. For a walker today it is the same idea: the flattest stretch of the whole canal, easy on the knees.