I guess before you start querying with Revenue whether you can claim certain tax credits it would be best to ensure you have exercised all your existing tax liabilities. You say you paid all monies due but were you claiming TRS at the ftb rate on your 1st property? Did you you pay stamp duty clawback?
You should be able to claim mortgage interest relief on the second property as a FTB as it's a time based relief (7 years) rather than one based on circumstances. However you can only claim it against your PPR, not against your Investment property.