Do I have to tell a private parking company who was driving in Northern Ireland?

This is different from a police Notice of Intended Prosecution, where the registered keeper can have a statutory duty to identify the driver. A private parking company is pursuing a civil parking charge arising from the terms of private land.

If you were not driving, you can state that fact and ask the operator to provide evidence that you were the driver. Do not name a false driver or provide misleading information. If you were the driver, whether the charge is payable depends on the circumstances and any valid appeal grounds.

Because the operator can still issue correspondence and potentially bring a Small Claims Court claim, keep all paperwork and respond to any genuine court documents even if you have declined to admit driver liability.