Employer not received RPN

No RPN (revenue payroll notification) has been received by employer and employee is stuck on emergency tax.

Is this the employer or employee responsibility to chase this up?

What are the most common reasons for this? Job not registered with Revenue?

Thanks

Usually the employer can register the employee on Revenue or via payroll. If the previous employer has already ceased the employee, usually the tax credits / cut off point etc transfer over automatically to the new employment.
The employee can also check online to see where their tax credits/cut off point are allocated and can set up a new employment, start date etc and allocate the tax credits there.
The employer cannot really chase it up due to gdpr, Revenue will not give any information as to why the employee is on emergency tax.

Is it the employees first job? They will need to set up the employment on their own Revenue myAccount. If it's not a first job they can check their myAccount as all tax credits will probably be allocated to their previous job if they or their previous employer haven't cancelled their employment yet. I set my son up as an employee last month and his previous job hadn't deleted him from their payroll. He was able to do it himself via myAccount.
As suggested, you won't get info from Revenue about an employees credits. I asked about one recently as there was no USC being charged. They said they couldn't tell me anything but did state the cut off point for the lowest salary for USC, which in a round about way made me realise that the employee was still exempt.

"No RPN" is a specific response from Revenue when a RPN is requested. Normally a RPN will always be (created) returned, it may contain zero credits and Cut Offs but that is a different issue. Possible causes are the employee (PPSN) does not exist on Revenue's systems or they/their spouse has a 'Case Worker Assigned', which is Revenues way of saying them are being audited. In any event it is the employees problem and the employee has to *phone* Revenue to get it sorted, or ask their spouse if they have been dealing with Revenue recently...