9,015 patients were left waiting for a hospital bed in the month of May.
According to analysis from the Irish Nurses and Midwives Organisation, the number is a 114% increase on May 2006 - when figures began.
78 of the more than 9,000 patients were children.
The INMO states that University Hospital Limerick and South Tipperary General Hospital recorded their worst May ever for overcrowding.
The 2019 figure for May is down on the previous year with 9,183 patients waiting on a bed in May 2018.
It is still up on the 2017 figure of 8,154.
Today's trolley watch figures show 336 are on trolley's waiting for a bed.
The INMO say 33 patients are waiting in the emergency department while 103 are in wards elsewhere in the hospital.
Cork University Hospital is the worst hit hospital with 41 patients looking for a bed.
Speaking about the May figures, INMO general secretary Phil Ni Sheaghdha said: "We are entering the milder summer months, when predictable seasonal illnesses are lower yet the level of overcrowding is worsening."
"Overcrowding in May 2019 is now at the same level as January 5 years ago.
"Clearly this is a capacity deficit and requires immediate investment in additional hospital beds.
"Considering the evidence of this continued increasing activity in our public health service, it is simply unacceptable that the HSE and Department of Health have introduced a recruitment pause.
"We know that this will simply lead to a chronic understaffing and overcrowding problem without any regard to person-centred solutions.
"Overcrowding and understaffing mean patients take longer to recover.
"That means worse care, higher costs, and greater risk of infection. Investment in beds and safe staffing is key to resolving this ongoing crisis."