You can get it back dated for up to 5 years i think. Just log into revenue on line. You can get the information there. You need to get the landlords pps number to claim.
The form you need to fill in is RENT 1.You can get €400 in extra tax credits if you pay €2,000 a year in rent.If you can't get the landlords pps number still send in the form.
You need to register with the online revenue site, you will recieve a pin number in the post, and this along with your pps number and mothers maiden name allows you to log in. You can claim rent relief, the amount varies on the amount of rent you pay, as well as stuff like refuse charges and medical payments.