How do you to calulate the Irish tax credit for UK tax paid

Does any one where I can get information on the calculations for allowances on UK Tax paid when your Irish resident. I've been working in the UK for most of this tax year but I hope to be back early next year so I'm still Irish resident. For income in the calendar year 2010 I will pay more UK tax than the amount due to the Irish. Can this amount be used to reduce my 2009 Irish tax bill. I don't see much work on the horizon in 2010 but the UK are only giving me three months allowances where at home I'm entitled to the whole year. Net effect is if is split the UK tax bill by time or Income I end up owing the Irish money in 2009 and overpaying with UK tax in 2010.

I have just gone through this because I have a UK pension. You need to obtain Form FD12 Double taxation UK/Ireland, from the UK tax office, you can obtain it on line or phone +44 151 2102222. Fill this in and send to your local Irish Tax office for approval stamping. They will forward it to the UK tax.
It is a very slow process as it takes 8 weeks after being processed by Irish tax.