Can't tell whether it's 6k per annum or 6k in total!
If 6k in total:
If you have less than €3174 of non-PAYE income p.a. then Form 12 should be sufficient.
Revenue generally don't apply interest to money owed by PAYE taxpayers, so just bite the bullet and fill out form 12 for years 2006-10 and pay whats due.
If 6k per annum:
This would suggest you should be filing a Form11 / Form 11E, and are a self-assessed person, so you could be hit with a surcharge, and interest on the tax due. If it's 6k p.a., then this could be a fairly big bill, and you should consider professional advice as to how best to proceed.