Interesting question. I'd say you won't be able to pay leas than zero tax.
Your income tax calculation is based on income after pension contributions (and after prsi/USC deduction)
Hence it isn't included in the income tax calculation in the first place.
So it isn't a question of "getting tax back" but of "not paying tax it in the first place"
Given that low income workers don't pay much tax, it doesn't change much
I don't think it is unfair to be honest. Any other pension benefits still apply eventually.
Mind, any tax benefits are also based on % if income, so if you earn 20K and are say 30 year old, 20% ie €4000 contributions would be allowed.
20% tax on €4000 means a theoretical reduction of your income tax by €800.