The normal cut off date for tax returns is the 31st of October unless you do the return electronically via Revenue Online Service (ROS) in which case it is today. However (and we never found out why) Revenue have recently determined that the 16th of November will instead be the final date. This means that anybody who has non-PAYE income to declare for 2010 still has a little time left to make the return on time.
If you don’t you’ll face a 5% surcharge on the tax bill up to December after which the surcharge rises to 10%. If you get the surcharge it means your rate of tax effectively rises by 10% (not absolute, but relative ie: 20% goes to 22%, 41% goes to 45%)