The Commission, which was set up in 1999, was initially given a two-year timeframe to complete its work and was expected to cost around €2.5m.
At the Public Accounts Committee today, Labour's Roisin Shortall asked the officials from the Dept of Education why the initial predictions as to the final cost were so wrong.
Ms Shortall said it was easy to get the impression that the department gave the Commission a blank cheque to carry out its work.
Secretary General of the Department Brigid McManus said the scale of the Commission's work had been underestimated as had the number of claimants who came forward to give evidence.
She also said a number of legal challenges also held things up.
