I’m not aware of any private sector pension schemes that allow you to purchase added years. It may have been possible in some such schemes many years ago, but I’d be astonished if there are any providing that option nowadays. If you are working in the Public Sector, then your HR department should be able to outline whatever options are available to you in this regard.
Assuming you’re not able to purchase added years, then you could pay AVCs to increase your benefits. These AVCs would be invested on a Defined Contribution basis i.e. the benefit you would receive would depend on how much you contribute and what rate of investment return is obtained on these contributions.
While AVCs can be paid on a lump sum basis, it’s more usual to pay them by monthly (or weekly, fortnightly) deduction from your pay. You will get income tax relief at source, but pension contributions no longer qualify for relief from PRSI or USC. Again, your employer’s HR department shoukd be able to provide you with details of the options available in this regard.