Revenue has published an eBrief setting out the Average Market Mid-Closing Exchange Rates v the Euro as supplied by the Central Bank. The eBrief also confirms that where an account was closed in the calendar year 2009, the conversion rate of sterling to euro should be calculated by reference to the sterling mid-closing rate supplied by the Central Bank, Stg £1 = €1.1260.
EBrief No. 06/10 is available here.