Can a driver sound the horn to tell someone at a zebra crossing to hurry up in Northern Ireland?

No. Highway Code rule 195 says drivers should not use the horn or wave pedestrians across at zebra crossings.

The guidance is designed to avoid confusing pedestrians, who may not know whether traffic in every lane has stopped. Wait until the crossing is clear and watch for people approaching from the side. Horn use is also restricted under the Code’s general rules; it is intended to warn of danger, not to pressure someone crossing the road.