I'd suggest you read through the existing information on this site, as all your questions have been answered already. Not being smart.
But to sum up some of the main points that might help. Note, for brevity I've left out some of the subtleties, but this is an overview.
1. Your pension will be calculated using the formula Final salary (averaged over 3 years) less twice the old age pension (around 25K), multiplied by (number of years working there) divided by 80.
In other words, if you have a €50,000 salary,, and you've been there for 20 years, your pension will be (50,000 - 25,000) X (20/80). So € €6,250 pa.
If you were there for the full 40 years, it would be €12,500 pa
2. Your tax free lump sum will be 1.5 times your salary multiplied by (years worked/40). So again, if you don't have the full 40 years worked there, your lump sum will be reduced accordingly.
3. AVCs can be used to increase your pension and tax free lump sum up to the revenue approved limits.
4. AVCs are tax efficient in that for every 100€ you pay in, it costs you 60€ net as you are saving tax, assuming you are on the 40% tax band. The AVC is taken out of your gross pay, and you see the net reduction in your take-home pay.
5. You are correct in that the old age pension is now payable from 67, but could, and probably will, change before you retire.
You will definitely have an in-house pensions person, talk to them and they will run the numbers for you. You can then decide if you can afford to pay AVCs and how much you want to pay.