Who should audit internal auditors?

The Internal Audit may be audited by:

1- Peer review audit.

2- Quality Assurance Review.

3- Independent Audit Firm

Audit Committee===============================

QA , and external auditors 

This Could be efficiently done by an independent  Audit Firm. Other entities can do this but the issue is  ANY OTHER INTERNAL COMMITTEES, inside the company, WILL not  be  INDEPENDENT . Thank You for your question .

external auditor

The answer is simple. It is the duty of external auditor to audit the work performed by Internal Auditor.

Secretary of the Audit department of the Organisation.