Central Bank figures show that a quarter of 786,000 residential mortgages have been taken out to finance rental properties, and more than half are on tracker rates.
Tracker rates move in line with rates set by the European Central Bank, and have been unprofitable for banks.
The figures were in a new quarterly report on the commercial and consumer lending markets in Ireland.
The figures show that Irish banks had €24.6 billion of outstanding buy-to-let mortgages on their books at the end of March, a quarter of all home purchase loans.
Tracker mortgages accounted for 53.4% of outstanding home loans totalling €52.9 billion, the report said. Such loans have caused significant losses at Irish banks as they tie borrowers to record-low ECB rates at a time when banks are unable to obtain cheap funding on the international money markets.
The total amount of loans to private households at the end of March was €155 billion. The volume of mortgages issued fell 0.6% in the first quarter, the fifth quarterly decline in a row.
