Do drivers joining a motorway in Northern Ireland have priority over cars already on it?

No. Traffic entering from a slip road must give priority to vehicles already travelling on the motorway.

Highway Code rule 259 tells joining drivers to check traffic and adjust speed so they can merge safely into the left-hand lane. A motorway driver may choose to make room, but the joining driver must not assume that another vehicle will brake or change lane. Use mirrors, signal your intention and match the flow without crossing solid white lines or using the hard shoulder.