A refundable security deposit is generally not rental income when a landlord receives it, because it remains the tenant’s money unless a valid deduction is made. If part of the deposit is kept and used for unpaid rent, damage or other permitted costs, that amount may have different tax treatment, particularly if it effectively represents rent. Any interest earned on money held may also need consideration. Keep clear records and check the current position with Revenue or a qualified tax professional, as the outcome can depend on the tenancy terms and how the deposit is handled.