You will be resident in a tax year if you spend 183 days in Ireland in a tax year, or 280 in total in a tax year and the previous tax year.
Therefore your timing of leaving and returning to Ireland is important.
However, in respect of employment income, if you leave Ireland with the intention not to be resident in the following tax year, you will be considered to be non-resident from the date you leave (if you actually do become non-resident).
So, you really need to stay out for a full tax year - go in 2009, and return in 2011, but you need to count the relevant days to make sure that you actually do break residence.