We are not discussing people smuggled to the UK
no need to smuggle
the common travel area borders are wide open
ireland does not share information with the uk
no controls on the land border
one can easily enter the uk without being checked by the uk authorities
nationality is not checked at the uk internal borders
interestingly ireland does not even collect API on internal EU flights
and their border controls are a joke
for eu citizens
when you buy an airline, train, coach, boat ticket for inbound or outbound to the uk all of the information you provide with your passport details are monitored, therefore the UK border agency have access to this data, I would also expect HMRC to have access to this data also.
hmrc relies on cameras at the borders as far as i know
for tax purposes
it is your obligation to prove your residency status
not theirs
so you should produce plane tickets etc if questioned