Lets say you earn 30k - this keeps you in the lower rate of tax, so you pay 20% tax on every additional €1 you earn, until your income exceeds 32,800. Lets say for argument's sake you pay €2,500 of tax on the €30k of income.
Now lets say they give you a €10k lump sum payment.
Now your total income is €40k and they tax the additional €10k as follows:
First €2,800 at 20% = €560
Remainder €7,200 @ 41% = €2,952
Additional tax = €3,512 (NB This equates to an effective rate of 35.12%)
This results in total tax payable of €6,012 (€2,500 + €3,512)
When you apply for relief they calculate the amount of tax you would have paid, if only one third of the lump sum was paid to you. In your case that would be a lump sum €3,333 and it would be taxed as follows:
First €2,800 at 20% = €560
Remainder €533 @ 41% = €219
Additional tax = €779
The effective rate under this special calculation is therefore €779/€3333.33 = 23.37%.
The income tax payable had you not received the lump sum (€2,500)
PLUS
Tax on the whole of the lump sum at the special rate (€10k @ 23.37% = €2,337)
EQUALS
Tax payable €4,837
Tax payable without the S480 relief was €6,012 and tax payable after applying the relief is €4,837 - the relief to be granted is the difference between the two - so €1,175 will be due back to you.
This is the difference between an effective rate of 35.12% and an effective rate of 23.37% on the €10k payment. (€10k @ 11.75% = €1,175).
As you can see, the effective rate can only be below 41% if the income without the lump sum was low enough that some of the payment falls to be taxed at 20%.