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In order to enter the UK as a visa national who is the family member of an EU citizen, you either need a "valid Article 10 residence card" or a family permit. In Ireland, the Article 10 residence card is implemented as the "Stamp 4 EUFAM". Since you don't yet have this, you should apply for a family permit. The permit is free and should be issued promptly.
Note that it is permitted, once you have the permit, for you to enter the UK through Northern Ireland without going through border control.