Tallaght Hospital, which has the third national children's facility on its campus, is owed just under €15m in total.
The Crumlin and Tallaght hospitals have joined with St James' to put out tenders for qualified and licensed organisations to provide debt collection services.
Between them the hospitals are owed more than €28m in unpaid patient bills.
Irish hospitals have paid out in excess of €800,000 in the last 20 months to debt collection agencies hired to chase patients for fees.
Beaumont Hospital has the highest spend on debt collection services, shelling out more €165,000 in a bid to recoup unpaid bills. The hospital is currently owed just under €12m in unpaid charges.
Overall, Dublin North East was the highest spending region, paying €378,630 for debt collection services.
Fianna Fail health spokesman Billy Kelleher said the issue highlights the disorder within the health system.
"I understand the need for hospitals to recover money they are owed, but the fees that we are seeing being spent on chasing debt certainly strikes me as excessive in the current climate," he told the Herald.
"The fact that the hospitals have been forced down this road is further evidence of the sense of chaos developing within the health system under the leadership of Minister James Reilly."
Stephen McMahon, of the Irish Patients' Association (IPA), said the amount of fees outstanding "calls in to question the management of the debts".
He added that his organisation has "credible evidence" that people are being chased for that they had already paid.
Mr McMahon said the €800,000 spent on debt collectors could have been better allocated within the health service.
"The one thing you don't want to happen is that parents would be reluctant to take their child back because there might be an outstanding amount," he said.