€478.78 per month is €5,746.36 annually
VHI is paid net of tax relief at source of 20% - i.e. your employer pays 80% of €5,746.36 to VHI, which is €4,596.
Your employer is supposed to treat the gross amount, the €5,746.36, as additional pay, and tax you accordingly. I think you'll find this is what they are doing, rather than deducting the €478.78, they're treating it as income and taxing it.
You then need to make a claim for the 20% tax relief in your own right.
So assuming you're a high rate taxpayer, you'll pay 52% tax, PRSI & USC on €5,746.36. That's an additional €2,988 in tax you pay during the year.
But you're entitled to €1,149 of tax relief (€5,746 @ 20%).
So the net cost to you. of your employer paying for your health insurance, is €1,839 (€2,988 - €1,149).
A damn sight cheaper than paying the €4,596 yourself. 60% cheaper to be precise.