Mullingar Regional Hospital was among five worst affected.
Nine times more people were on trolleys in Irish hospitals last month than during April.
According to the Irish Nurses' and Midwives' Organisation, the number rose from 497 in April to 4,499 in October.
Mullingar Regional Hospital was among the five worst affected facilities, with 287 people waiting on a trolley last month - while the highest figure was recorded at University Hospital Limerick, at 1,064.
The INMO says the rising numbers are very concerning, as the winter season begins.