Belated response but I have come to the conclusion that, in fact, they give you relief of 32%, ie not 40% as is implied.
Say that on the marginal €12,500 of your annual PAYE salary, you are deducted 40% in income tax, 8% in USC and 4% in PRSI.
So you are deducted €6,500, comprising €5,000 tax and €1500 in combined USC and PRSI.
This would leave you with €6,000 to put in your pocket or €500 every month.
Instead of taking the money you decide to have it put into AVCs every month.
At the end of the year you will have put in €6000 and the government will put in €4,000 in refunded tax
(because they calculated that you had paid 40% income tax on an imaginary €10,000 gross).
So they are with verbal sleight-of-hand, diddling you out of the €1,000 you actually paid in income tax on the extra €2500 above €10,000.
The €1,500 PRSI and USC have to be paid in any case as you say but not considering them when working out the tax refund leads to an incorrect result.