Revenue publish a chart of [broken link removed]for each year. This can be found also in Revenue's publication "Tax Briefing". The amounts filed on a €uro Return should be given in €uro.
Revenue accepts different methods of calculating excahnge rates, as long as the methods are used consistently.
For example, if you use an average rate for the year, you should use and average rate next time you calculate it for revenue and not change to a historical date.
This is my understanding.