On TRS you can claim if the purpose is 'purchase, repair, development or upkeep' of PPR. That also applies to 'top ups'.
If you are borrowing for an investment property, its usually better to structure the borrowings against that property, though if done by way of equity release on PPR that interest should also be deductible against the Rental Income.
The Revenue Guide to Rental Income - which Revenue will say is only a guide - only makes metion of monies borrowed and does not specify that this is the specific property or otherwise.
[The only reason I mention this is that there is some doubt about interpretation ... as a UK tax case saw a situation where the taxpayer was able to deduct the interest on borrowings secured against an investment property regardless of the purpose]