More than four in ten home loans sold by Start Mortgages are over three months in arrears.
The figures are based on a pool of mortgages resold to investors by Start, according to ratings agency Fitch.
Start resold two pools of mortgages called 'Lansdowne 1' and 'Lansdowne 2'.
Fitch downgraded both today. It expects Irish house prices to fall 45% from peak to trough.
For 'Lansdowne 1' loans of more than three months in arrears represented 38% of the total and the figure for 'Lansdowne 2' was 41%.
The ratings agency published figures for Bank of Ireland showing a pool of its mortgages had loan arrears of 3.3%. A group of loans sold by Ulster Bank had arrears of 2.4%.
