Basically you have to pay the income tax on rental income (and other none PAYE income) in advance. So you pay the tax on the 2010 income in Nov 2010, etc
Thus you have to estimate what your 2010 income will be and pay it in November 2010. Obviously, you didn't do this in 2009 as it was the first year of rental income. So now you are paying for 2009 and also your 2010 preliminary.
In 2011, after submitting your 2010 tax return you will have to pay some more, if your estimate was too low, or you will get a refund, if your estimate was too high.
The estimate can be based on a number of options but if your estimate turns out to be too low, the Revenue can apply penalties. In general, the estimate for 2010 will be based on the 2009 figure but you are at liberty to make your own estimate, with the provisio that too low an estimate can leave you open to penalties and late payments interest.