Can cyclists use a footbridge marked for pedestrians only in Northern Ireland?

Not as a cycling route. If cycling is prohibited, dismount and walk where pedestrian access is permitted.

The sign at the entrance determines whether a bridge or underpass is shared-use, segregated or pedestrian-only. Do not assume that a safe-looking bridge is authorised for riding. On mixed-use facilities, give pedestrians room and watch for tactile guidance intended for visually impaired users.