It is a legal requirement that if you rent a house to a tenant in the Republic of Ireland all such tenancies should be registered with the Private Residential Tenancies Board.
There is a fee for registering every tenancy. If a landlord does not register a tenancy the landlord may be guilty of an offence and may be liable for a fine of up to €3,000 and or up to 6 months in prison.
If tenancies have not been registered with the Private Residential Tenancies Board any mortgage interest you pay on loans to purchase the property are not deductible for tax purposes.
The Private Residential Tenancies Board website www.prtb.ie contains further details of rules and regulations as well as the registration process.