These figures come from the Stability Programme Update for 2006. The data methodology is in there, but basically:
The output gap figures are used to calculate the Cyclically Adjusted Budgetary Balance (CABB), which is used to assess the ‘close to balance’ budgetary requirement.
Which is a much better explanation than I could give.
2004 2005 2006 2007 2008
Real GDP Growth 4.5 4.6 4.8 5.0 4.8
Potential GDP Growth 5.8 6.1 5.5 5.3 4.8
Output GAP 0.1 -1.3 -2.0 -2.3 -2.3
And here you can download the spreadsheet I used to make the graphs.