I wonder what kind of border Nigel Pike envisages in Ireland. Maybe one that is now referred to as the internal border in Germany that once divided the East and West. It had watchtowers, fences, mines and armed guards ready to fire.
In fact both the British and Irish governments have recently signed an agreement which will protect the existing rights of each country’s citizens to live and work in either country. In other words, both governments want to maintain an open border in Ireland regardless of any kind of Brexit.
Is there a massive flood of illegal immigrants across the border? The answer is no. Both the Irish and British immigration authorities cooperate on making checks, and to travel from Northern Ireland to Britain by air or sea requires some form of identification to be shown.
Will the population increase in the Republic? Of course: banks and other financial companies are relocating to Dublin because of Brexit. The British are the second-largest group of immigrants in the Republic of Ireland, the first being the Catholic Poles. Lastly the criminal gangs in Ireland are mostly locals, and the gardia (police) are after them.