There are, at present, three economic borders with respect to the island of Ireland. First, there is what we might call the “Celtic Sea border”, between Ireland and the rest of the EU. Ireland is in a common travel area with the UK and is not in the Schengen area, so there are in principle passport checks between Ireland and the rest of the EU at the Celtic Sea border.
Second, there is what we might call the “Irish Sea border”, between Great Britain and the island of Ireland. There are currently controls on the importation of live animals into Northern Ireland from Great Britain.
Third, there is the “North-South” border between Northern Ireland and the Irish Republic, including some 275 crossing points. This border (either side of which there are a large number of regulatory, tax and legal differences) is controlled via a combination of administrative cooperation, whistle-blowing, auditing, site raids by customs, tax and regulatory enforcement officials, occasional random spot-checks on roads leading up to and at the border (yes, that happens now), and cameras and other physical infrastructure at the border (yes, that is there already).