Please stop panicking! There's no Keeper liability in Northern Ireland. There is no legal obligation on the Keeper to identify the driver to an unregulated private parking company. ECP have no idea who the driver is unless the Keeper blabs it to them, inadvertently or otherwise.
You were absolutely correct to just ignore the NtK. Stop getting your knickers in a twist about useless debt collector letters. Ignore them. They are powerless to do anything because they are not a party to the contract allegedly breached by the driver. Use their letters as emergency toilet paper if you feel the need.
Never, ever, EVER communicate with a useless and powerless debt collector. They cannot do anything except try and get the low-hanging fruit on the gullible tree to pay up out of ignorance and fear. Just ignore them and please don't show us their useless letters.
No one who receives an ECP PCN pays penny if they are here receiving advice and following it. You will be no different and you have the added advantage that you are in NI which means that as the Keeper you cannot be liable for the charge. Only the driver would be liable and you have not told them who that is because you are not obliged to so so.
Just continue to ignore them. ECP cannot validly issue a claim in an English court for a parking contravention in Northern Ireland involving an NI resident. If they attempt to do so, the claim is open to jurisdictional challenge, which would result in its dismissal. They may be stupid, but they do not like wasting too much of their money.
It would be prohibitively expensive and logistically challenging for ECP to issue a court claim in NI. If they ever did, and you'd be the first in recorded history, you can get help from the Northern Ireland Consumer Council. They can provide advice and assistance to residents, including matters related to parking disputes. The Consumer Council for Northern Ireland is an independent organisation that helps consumers resolve issues and ensures their rights are protected.
A caveat though... just like the Citizens Advice Bureau (CAB) here in England, they won't be as exert on private parking matters as we are here. So, if you receive any advice that conflicts with what I've told you, come back and tell us so that we can make sure you don't make any stupid mistakes. As I said, no one pays an ECP PCN if they follow our advice.