As a bit of advice, if you are claiming to have a home-office. Don't claim for an exact proportion based on floor-space - Dublin66 does have the right idea but you should then restrict it again by perhaps 1/2.
Heat can be done on square metres over total - but you need to further account for the hours per day as a % of total day. Eg if you had a 700sq m home and used 100sq m for your work and worked 50% of all available hours and your bill came to 1,000EUROS, then 100/700*50% = total tax deduction.
Why is this latter 50% apportionment important? CGT on your PPR is denied to portions of your home used exclusively for business. Claiming a % for the entire 'space' would add to the Revenue's argument in event of a dispute. (There are other ways around this too - putting the space to other uses in the evenings)