You'll need a local accountant in the Czech republic to file your returns there first. I'd imagine that you have until March 31st 2009 to do this (you should double check this date), but you should try to sort the situation out as soon as possible, as you'll probably need a local tax number, permission to rent, etc.
Following this, you'll need to hire an accountant in Northern Ireland (or self-assessment if applicable) and work out if you need to pay any further tax in your own jurisdiction, deducting the tax already paid in the Czech republic.
Both accountants should be able to advise which deductions are permissible in each jurisdiction.