You really don’t seem to be familiar with Irish immigration procedures and set up.
Both governments share data with each other about everyone who enters either jurisdiction through ports and airports from outside the two countries.
That is not going to change.
The notion that mass illegal immigration is going to happen across the border is ludicrous as UK Border Force have the power already to stop potential illegal immigrants already by way of intelligence led spot checks and do so as it is.
Non-EU entrants to Ireland holding an Irish visa already do not necessarily have the right to enter the UK, as visa requirements are different for the two countries. That is policed by intelligence led checks at UK airports on flights from
Ireland and within NI by internal checks.
Firstly I'm very aware of Ireland/UK's intelligence, I was one of the few - if possibly the only one before the referendum - that recognised this issue before the referendum and spoke to my MP (Green, Bolton W) and was also on the BBC to point this out. I called it correctly.
Okaaay...let's run this through you. I'm Pavel and I'm a convicted polish criminal. I go to Ireland, and the Irish authorities scan my ID card and tell the UK I'm in Ireland. I check in at the Holiday Inn in Dublin. 7 nights.
On the third day I decide to catch the train, using cash, to get to Belfast - in the UK of course.
How on earth are the UK authorities gonna stop me? There's no checks on the train, and if they did follow me (how much would THAT manpower cost??) how do they know I'm not an IRA sympathsizer who's armed to the back teeth should I be stopped?
Mass immigration across that border WILL happen because it will be the only route in for those who want to do us harm, or those who want to join our post-Brexit golden highlands and want a share.
And non-EU immigrants is a total red herring as we are concerned about EU migrants - that's the EU you want to leave.