There is an interest charge of roughly 12% per annum on underpaid tax. The Revenue also have a raft of penalties available to them, and you can expect to be hit with an additional 10% of the tax charge for submitting your 2005 tax return late.
Your 2006 tax return is due by 31 October 2007.
Your 2007 tax return is due by 31 October 2008.
Get your returns in immediately and pay the tax due (including the 10% surcharge on 2005). Then wait to for the bill for the interest (it's non cumulative, so paying the tax amount will stop the interest accruing).