See here:
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In 2007 (for a single person under 55) a maximum of €1,800 rent qualifies for relief at 20%. So the maximum benefit in terms of hard cash into the individual's hand that would otherwise have been kept by the taxman is €1,800 @ 20% = €360.
You can backdate claims for outstanding tax relief by a maximum of 4 years - i.e. going back as far as 2003 right now.