ROKNALets get some things clear
1. If you are anything other than a UK or Irish national you need a passport or national ID card
2. Everyone arriving in Dublin Airport will go through immigration, no passport = no entry unless you have a Irish or UK drivers licence which shows you to be Irish or UK national and arrived on a UK flight
3. They do check the trains and buses on a regular basis, not as often as before but it does happen
4. Travelling Ireland -> UK you go through the domestic channel and bypass all immigration though legally a non UK/Irish national should report to the immigration service at that airport
All correct however lately i have noticed a change to arrival into the UK.
A couple of airports i have arrived into from the ROI over the past couple of months or so, now have a seperate area dedicated to ROI flight arrivals. While it is not the usual UK immigration officers there checking documents it is UK border police officers. Other airports seem to still have the UK domestic and Republic of Ireland arrivals section but also are covered by border police officers.