Apart from providing rich data security information, database audit trails are the good sources of information on the workload characteristics of database system. This is why the periodic patterns of database operations found in the audit trails can be used for both performance tuning and intrusion detection. This work defines a new concept of an irregular periodic pattern and it shows how such patterns can be detected in the audit trails of database operations. The paper presents the algorithms that implement a new way of finding the irregular periodic patterns in audit data. The quality and complexity of the algorithms together with the outcomes from the experiments are discussed in the paper.