The head of the HSE will appear before the Public Accounts Committee later to answer questions about allowances for public servants.
In 2011, just over €122m was paid in allowances at the Executive, while around €170m was spent on overtime and almost €257m went on night and weekend work.
Total pay for the HSE last year was over €5bn and of the almost 68,000 staff employed there, nearly 42,000 were in receipt of an allowance of some kind.
Tony O'Brien will face questioning on the 41 allowances payable to HSE staff.