As per Revenue.ie
"Prior to 5 December 2007, the period where no rent could be obtained was five years. Under the Finance Act 2008 this period has been reduced from 5 years to 2 years for deeds executed on or after the 5 December 2007.
For instruments executed before 5 December 2007, to the extent that a dwelling house or apartment is rented out on or after 5 December 2007, it will not involve a clawback of the relief where this occurs in the third, fourth or fifth year of ownership."