Or there is the distinct possibility that this was a heavily-traversed border pre-Brexit and with Brexit it now requires a lot of border checks.
With time countries usually end up passing treaties to make these regular passings easier. When you are a country newly enforcing the Schengen border it’s a high faff affair and closely scrutinized.
I remember when Slovenia first got the border it caused huge problems and long delays.
The checks are at the ports in NI, the whole point of the agreement between the EU and UK was to avoid any checks at the land border between NI and the Irish Republic.