I live in the UK. I occasionally do work for Irish clients (from my home office), and am paid in Euro into an Irish bank.
When including such earnings in my UK tax form, I use the 2nd method Domo suggested, i.e. the average yearly rate published by HMRC on their web site. A while back, when the exchange rate was steady, this was fine. Now that Sterling has fallen in value, I'm not sure if the HMRC average favours me or them - suppose it depends on which part of the year most of the income was earned!
I also complete an Irish tax return, for the rental income I earn in Ireland, and this also has to be declared in the UK tax form (the "foreign pages" section). Main problem here is that Irish and UK tax years are different. What I do, for example, is include my 2007 Irish income in my 2007/2008 UK tax return. I get my Irish tax assessment before completing my UK tax form.
James