Taxes Consolidation Act, 1997 (Number 39 of 1997) 739X Application of this Chapter This Chapter shall apply to— (a)accounting periods commencing on or after 1 January 2017, or (b)where an investment undertaking's immediately preceding accounting period ended on or after 31 December 2015 and a decision was made after 20 October 2016 to change the accounting period such that paragraph (a) would not apply, that accounting period commencing on or after 20 October 2016.