You may need to ask your Solcitor for the number. She or he should have completed form 901 to register the property in your name. This should have been followed up with form 902 (from her or the builder) with a copy of the house plans.
On the 901 form copy, at the top, there should be a Catastral number. It’s usually about 14 digits long made up of numbers and letters.
If neither of these forms have been submitted then your property is probably not legal at all. If your solicitor doesnt have these details the next step would be to go to the town hall and ask for your catastral number.
If you submit a 901 form, they have to register it, stamp it and give you your catastral number.