If an employee is employed by an ROI company but based in N.I. under whose employment law jurisdiction do they fall for paternity leave for example? Alternatively, if the employee is employed in NI but works in ROI, which level of national minimum wage (NMW) would apply, for example?

The question of the governing law applicable to a particular employment relationship depends on a number of factors.

Usually, a contract of employment will contain an express governing law clause which states the applicable law. However, even where a contract of employment includes an express governing law clause, case law from the Court of Justice of the European Union and EU Regulations provides that the mandatory rules of a different country may apply, if the contract has a "close connection" to that other country. Mandatory rules include such entitlements and statutory protections that the parties cannot contract out of. 

The practical implication for the employer is that, even if