Your employer is charging you tax and PRSI on the BIK attributable to them paying your VHI premiums as they are legally obliged to do. You can also claim a tax credit in respect of the premiums which will mitigate your deductions a bit:
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Maybe the BIK was backdated this month but you will pay BIK tax and PRSI as long as you are availing of the employer payment of VHI contributions. If your premium is €45 p.a. then you can generally expect to pay tax (20% or 41%), PRSI (4%) and health levy (2%) on that amount. If you claim the employee/personal tax credit in respect of these premiums then this will reduce your tax/PRSI liability a bit.