Do mandatory variable speed limits on Northern Ireland motorways have legal force?

Yes. A variable speed limit displayed inside a red circle on an authorised motorway signal is a mandatory maximum, not merely advisory.

Northern Ireland Highway Code rule 261 requires drivers to obey a lower speed limit in force on a motorway, including a mandatory limit displayed on an overhead or roadside signal. This is different from an information sign warning of congestion or recommending a speed. Reduce to the displayed limit in good time and continue to comply until a different lawful limit applies.