“Then just drive over the border and illegally enter the UK. It would become like our border with Mexico, a magnet for illegals and illegal activities.”
Northern Ireland isn’t known for a strong economy. It’s one of the weakest in Western Europe.
Things have changed there since 1912, when ships like the Titanic were built in Belfast.
Not quite. Although the Republic is in the EU, it's not in the Schengen open-borders zone. (The UK isn't either). This means that anybody flying to Dublin from elsewhere in the EU has to pass through passport control. Existing EU citizens have automatic right of entry: but anybody else only gets in if the Republic wants to let them in.
There's a separate bilateral Common Travel Area agreement with the UK: so that arrivals from the UK, alone among EU countries, don't have to pass through passport control.
Ireland is already outside the EU’s Schengen agreement that gives open borders between EU states who are part of it, but they are in a common travel area with Britain and have been since independence. Unfortunately, Ireland are already even more useless than HM Immigration services when it comes to protecting their borders. If it were up to me I’d hand NI over to the republic (whilst granting NI residents the right to British citizenship) and then scrap the CTA and the rights and privilages granted to Irish citizens that make them virtually indistinguishable from British citizens. Its long past time we started treating Ireland like the foreign country it is and has been since 1922.