Monitoring the unregulated land border between the Republic and Northern Ireland is the responsibility of the Belfast unit of the UK Border Force. The unit has a total strength of about 60, but the bulk of it is dedicated to the three primary control points at BFS (and during the summer at LDY).
As far as the land border goes, they prioritise the detection of illicit commodities flowing inward over the detection of illegals. All the rest of the information about their detection operations is closely managed and unavailable (even with Freedom of Information). When they provide evidence to the Home Affairs Committee, anything having to do with detection operations is snipped out.
However, they have given evidence that suggests the largest part of their border operations is in reaction to intelligence received from the field and the largest part of that is normal, every day, 'ordinary mortals' providing tip-offs. We also know that they have a system of about 300+ points where cameras are installed along the land border.
Per the comment from tricasse (to whom thanks) citing the Wiki article, they do not like to impose a large presence on the land border for political reasons. As a counterbalance they have a partnership with the local constabularies where they get alerted to the need for detection staff.
Per my own comment above, you can find the Chief Inspector's report at Inspection of the UK Border Agency in Scotland and Northern Ireland: Border Operations. The Agency replied to this report at The UK Border Agency Response...
Related question: How long before I can re-enter the UK after Youth Mobility?
For your own comment: "I'm going to Belfast for important but risky personal matters, and would prefer to leave my ID card at my Dublin hotel to eliminate the risk of it getting stolen." You are overthinking it, but of course that strategy is lawful and should not present a big problem if you are challenged.