Thanks for that PBC, and thanks for not getting mixed up in davie's and fender's sexual fantasies.
You are right the border between NI and Eire is lightly controlled, even non-existent. Neither Eire or UK is in Schengen area, so they are free to control their external borders to a high degree. This is due to an agreement between UK and Eire in 2008.
During the 1916 to 2008 period there has been several changes in land border control, due to various reasons.
However, the UK and Eire governments agreed to cooperate on their external borders, so leaving the internal borders open.
In July 2008, the UK and Irish governments announced their intent to resume controls over their common border and the Common Travel Area in general. Each proposed to introduce detailed passport control over travellers from the other state, where travel is by air or sea.However, the land border will be 'lightly controlled'.
https://en.wikipedia.org/wiki/Republic_of_Ireland–United_Kingdom_border
This is exactly the type of arrangement that EU is trying to replicate, but sadly they have paid too little attention to the external borders, leaving it to the individual member nations. This is exactly the argument that I have been making: the concentration and cooperation should be on the external borders. The internal borders can then become open, allowing free movement, total convenience for the citizens, reduced expenditure, improved international cooperation, etc.
Edit: BTW, it also highlights how a land border would need to be recreated in the event of Brexit.