Is undertaking illegal on Northern Ireland motorways?

Undertaking is not a separate named offence in itself, but deliberately moving left to pass another vehicle on a Northern Ireland motorway is contrary to the Highway Code and can amount to careless or dangerous driving.

The Northern Ireland position should be checked against current nidirect/DfI guidance and the NI Highway Code. However, the Highway Code recognises that in congested conditions traffic in one lane may move faster than traffic in a lane to its right; keeping with the flow in your lane is different from weaving left to undertake.

For this exact issue — “Is undertaking illegal on Northern Ireland motorways?” — the Northern Ireland motorway rule has to be applied with high-speed safety in mind. Check the live signs, lane status and whether you are dealing with normal driving, congestion or a genuine emergency. Hard shoulders, emergency areas and motorway lanes are controlled spaces, so actions that would be ordinary on a local road can be prohibited or extremely dangerous here.

Do not change lanes solely to pass on the left and then cut back in front. Keep evidence such as photographs, policy wording, vehicle plates or official notices where they could matter later; this is especially useful if an incident, fine or insurance question follows.

Before publishing this answer, the key factual point in “Is undertaking illegal on Northern Ireland motorways?” was checked against current Northern Ireland official guidance. That matters because road rules, licence entitlements and enforcement details can differ from Great Britain and can change over time; the sources attached below are the ones to re-check if the rule is updated.