This describes the most common case:
Example 1
The employer pays the full medical insurance
premium for an employee
The renewal notice to the employer shows the net
premium due - £1,600 (Gross Premium is £2,000)
Employer
The employer pays the insurance provider the net of tax
relief premium of £1,600. He/she pays the tax relief at
source (£400) attributable to the premium to the
Collector General. The employer operates PAYE/PRSI on
£2,000 notional pay to the employee.
Employee
As the employee has not benefited from the tax relief
arising on the medical insurance premium paid by the
employer, the employee is entitled to a tax credit of £400
(£2,000 at 20%) in his/her tax credit certificate.
Revenue
TRS Section in the Collector General’s Office pays over
the TRS of £400 to the medical insurance provider.
See
http://www.revenue.ie/en/tax/it/leaflets/it1.html#section9