The house price index compiled by Permanent TSB and the Economic and Social Research Institute shows that falls in prices accelerated as the year went on, with a drop of 3.6% in December alone.
The 18.5% fall in 2009 compares with a fall of 9.1% in 2008.
At the end of last year, the average house price was €213,183, compared with €261,573 at the end of 2008.
The index shows that prices have fallen by 31.5% since the peak in February 2007.
Permanent TSB's Niall O'Grady described 2009 as a 'horrendous' year for the housing market, due to 'shattered' confidence, oversupply and a weakening economy.
He said an 8.5% fall in prices in the last quarter of the year was more than twice as big as the drop in any other quarter since prices started falling.
