Is motorcycle filtering through slow traffic automatically illegal in Northern Ireland?

No. Filtering is not automatically prohibited, but a rider must do it with care and remain in proper control.

Highway Code rule 88 specifically discusses filtering in slow-moving queues, telling riders to keep speed low and watch for pedestrians between vehicles, vehicles changing lanes and traffic emerging from junctions. A rider does not gain permission to cross solid lines, ignore signals or put pedestrians in danger simply because traffic is queued. Check mirrors and blind spots and avoid filtering through a gap that could close suddenly. The legality of a particular manoeuvre depends on how it is performed and the road controls present.