Can you sound your horn while stationary in Northern Ireland?

Generally no. The Highway Code says drivers must not sound the horn while stationary on the road.

Rule 112 also limits horn use in a built-up area between 11.30pm and 7.00am, except when another road user poses a danger. A horn is a warning of your presence, not a tool for expressing impatience at traffic lights or queues. If another driver fails to notice a green light, avoid treating repeated horn blasts as a routine solution; wait and consider whether a genuine safety warning is needed.