National Insurance letter code for overseas employee

We paid employer national insurance in Ireland for one of our employees but we also paid it in the UK and are now seeking a refund of the NI paid in the UK. I have been advised by the employer helpline to adjust the FPS however I need to know which National insurance letter code to use - should this be X? Thanks

Hi,

You can use catagory X if the employee is not liable to national insurance in the UK.

Thank you.