The HSE spent over €8m rehiring retired staff in a single year, according to figures released by health officials.
The controversial practice, which breaches the HSE's own recruitment policies, resulted in 514 former staff being rehired in 2013 at a cost of €8.4m to the taxpayer.
Despite breaching its own rules, the HSE defended the filling of the positions with former staff, saying this was only done in "exceptional circumstances".
The rehired staff included 266 retired nurses who were pressed back into service at a cost of €3m.
In a briefing note for TDs on the Dáil Public Accounts Committee, health officials said it had been necessary to rehire certain categories of staff on a short-term basis due to difficulties finding suitable replacements.
It said urgent service needs or recruitment processing requirements meant the staff were required in the short term to support on-going services.
These included medical consultants who were asked to stay on after retirement to cover their previous posts.
Retired psychiatric nurses and other skilled nursing staff were also rehired "where particular sourcing challenges exist".
Officials said pension abatement rules kicked in for all of the rehired staff, meaning they could not be earning on the double.
Among those rehired were 80 medical staff at a cost of €3.2m, 87 support staff at a cost of €878,000 and 23 health and social care workers at a cost of €577,000.
Some 30 former managers or administrators were also rehired at a cost of €352,000, while three dental staff were rehired at a cost of €74,000.
The rehiring of former staff has been a persistent issue in the HSE for several years and had continued despite instructions being issued to managers in 2010 to cease doing so.