It comes as a record number of people attended emergency departments last week, with over 30,000 presenting for care.
The Irish Nurses and Midwives Organisation say there was 85 patients without a bed at CUH and the Mercy today.
The HSE is also dealing with the annual rise in flu and Covid cases, along with RSV - a virus found in children.
Interim chief executive Stephen Mulvaney says cancelling scheduled care is to be expected:
"We generally get to a point in the year, often as I said it's in early January, when we do need in some sites to curtail the level of planned activity. And that decision is not made lightly but it's done in terms of managing overall risk. What we are going to do right the way to the year end and as far as we can into the new year, is to drive on with the efforts to improve our scheduled care."
