There are currently no border controls between Ireland and N.Ireland or the island of Ireland and the island of Great Britain.
https://en.wikipedia.org/wiki/Republic_of_Ireland%E2%80%93Un...
https://en.wikipedia.org/wiki/Republic_of_Ireland%E2%80%93Un...
Here is the law:
http://www.irishstatutebook.ie/eli/1997/si/277/made/en/print
Checks by air and sea are authorized and implemented. The UK Border Force also performs spot inspections.