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The status is known as deemed leave. Deemed leave is statutorily held and cannot be granted or refused; in fact even if admitting intent to overstay or work illegally, entry cannot be refused. However, several cohorts of people are disqualified from deemed leave, and must seek out a border officer (unless stopped in the first place) and be assessed in the ordinary way; such people are consequently prohibited from entering at the Irish land border (as the UK never checks there) unless obtaining a visa in advance (in Dublin or elsewhere)
See the comments about non-visa nationals in the section βTravel Ireland - UKβ in this answer What do non-EU/Schengen citizens need to do when travelling within the Common Travel Area (CTA)?