A good rule of thumb:
Assuming you have no other pension benefits already would be to make contributions equal to a percentage of your salary that is half your age.
So a 27 year old would need to put away about 13% of their salary.
This is the sort of contribution you would need to make to get 2/3rds of your final salary at normal retirement with inflation protecton and spouses benefits assuming a conservative annual growth rate - say 6%pa. Don't scoff, over the last 10 years, the average Irish pension fund has grown by 4%pa according to Mercer.
Scary isn't it? Nearly everyone underestimates the level of contribution required to provide this level of pension. Just a few short years ago, it was normal for a employer to offer a defined benefit pension (sometimes with no contribution at all required by the employee) to provide these benefits.
With the dramatic collapse of the defined benefit pension, in less than a generation, the risk of making provision for our old age has been transferred from the employer to the employee but without any increase in our understanding of the issues at stake.
A final thought: The amount you need to save doubles roughly every 5 years just to stand still. So, to get the same end result as a 20 year old saving €100pm a 25 year old would need to save €200pm. Just to get the same end result!
Start early. Save as much as you can, increase it when you get a pay rise and keep on top of the investments. If you don't understand investments, get really (and I mean really) competent advice.