Nearly 6,000 complaints about financial institutions were made to the Financial Services Ombudsman last year.
This is an increase of more than a third on the number received in 2007.
Financial Services Ombudsman Joe Meade has said he is concerned that some Credit Unions might be exposing members' funds to unacceptable risk.
Mr Meade received 5,950 complaints last year - 2,600 were against credit institutions while 3,300 complaints were about the insurance sector.
Almost one in six complaints were about investment issues - up nearly 2.5 times on the 2007 figure.
Mr Meade has contacted the Registrar of Credit Unions in the Financial Regulator's office in relation to how credit unions invest money after one credit union signed up for a deal which lost €1m of members' funds.
He said that on the basis of that case, he was concerned about how Credit Unions and the brokers who were advising them might be operating and might be exposing members' funds to 'a degree of unacceptable risk'.
In relation to another case involving the incorrect identification of a bogus non-resident account holder, the Ombudsman re-iterated his policy that he would not compensate anyone for tax unpaid, concealed or understated.
Mr Meade said that while it was not in his remit to investigate the business activities of Anglo-Irish Bank, two people had asked him to investigate the activities of Sean FitzPatrick.
Mr Meade said: 'The fact that another financial institution gave him the loan in order to hide this from everybody is a cause of concern.
'The fact that complaints to me have risen by 36% might be an indication that the culture over the last few years, in which the financial services sector operated, was not as good as it should be.'
