Yes, you can pay a backdated contribution for 2008 and claim relief against your 2008 income, provided you do so before the tax filing date for 2008 (end October 2009 or mid November 2009 if using ROS). You can also claim relief against your current year's income in respect of your regular monthly contributions.
Your employer is obliged to facilitate you making the regular contributions through payroll out of your pre-tax income, either through a company pension scheme or a PRSA. By adopting this approach, you will also automatically get relief from income tax and PRSI on your regular contributions and your employer will also get relief from employer's PRSI.
The backdated contribution will have to be paid gross (i.e. out of after tax pay), but as you are a PAYE taxpayer, you can claim relief from PRSI as well as income tax.
If you exceed the PRSI threshold for the year, the PRSI relief you get will effectively be at 2% (2.5% if your income exceeds €100,100).
Regards
Homer