I'm not 100% sure about whether CGT arises but, if it does, you would apportion the gain since you bought the house against the proportion of time you had it as a PPR vs "investment property". So if you bought the house in August 2004, moved out in August 2007 and sold in August 2008 and the difference in the house price (ignoring indexation) was €100k, then your gain would be €25k (1/4 of €100k). From what I gather from other posts on this topic, the difference in rate of increase over the years (e.g. the €100k gain may have occurred prior to August 2007 and house price may have remained static since) is not taken into account.
Sprite
p.s. posts crossed with clubman