You are mixing things up
1. Tenant's or agent's have to deduct 20% from the rent and remit it to the Revenue as a prepayment of tax due where the landlord is non-resident.
2. The owner has to submit a tax return each year showing the net profit, if any, made on the rental and pay tax at the appropriate rate. Some, but not all, capital expenses are allowable expenses when calculating the net profit and are written off over 8 years. Any tax remitted by the tenant or agent is offset against the tax due on the net profit
These are two separate calculations and, indeed, are carried out by two different people