It would be worth keeping them for 6 Years.The Revenue people can go back this far for routine checking of any tax claims for refunds,i.e. medical expenses claimed over this period.Of course if you had a P21 assessment for each of these 6 years in all probability all is perhaps in order. It is worth checking payslips to see that all Cumulitative pay SRCOPS Tax credits and PRSI weeks with the correct contribution Class are in order.It is possible,but most unlikely,that but there may be Minor errors. I'm not so sure about the Pension contribution part of your query.I do not know who you would have to prove the deduction of contributions to? The P60 will only show the Taxable Portion of your Salary, e.g. Salary €40,000.00c less contribution to pension of say 20%.This amounts to €8,000.00c, so the Taxable portion of your salary is €40,000.00c less €8,000.00c i.e €32,000.00c, you will only pay tax on €32,000.00c at whatever your Marginal tax Rate is.Please do bear in mind that the Full Salary is Liable to PRSI/USC deduction without any relief at all. As stated the P60 will only show €32,000.00c,but it will state that this is salary less pension contributions.
See (A) PAY Paragraph 1 of P60. I hope this is of some assistance to you