Garda Commissioner Martin Callinan has said there are 25 organised criminal gangs operating in the State.
Speaking at the Oireachtas Justice Committee, he said the majority of the gangs are based in Dublin, Cork, Limerick, Galway and Sligo.
The commissioner said a number of the gangs have a significant international dimension and some interact to pursue joint enterprises.
The majority are involved in drug-trafficking and are prepared to use violence and intimidation.
He said each gang has a structured hierarchy with a leadership, a number of middle-managers and low level criminals who carry out the day-to-day running of the gang.
Some gangs have links with international criminal networks in Holland, Spain and the UK.
Commissioner Callinan said gardaí are investigating five gangs with significant cross-border connections.
The committee was told that gardaí have observed associations between Irish and Russian organised criminal gangs who work together on drug and cigarette ventures.
The presence of Russian gangs operating in Spain is also influencing the activities of Irish criminals there.
Links between dissident republicans and criminal gangs continue to cause serious concern.
The commissioner said he does not want to see garda numbers go below 13,000, and that he would like to see the embargo on recruiting lifted.
Separately, the commissioner praised the Garda Fraud Bureau and the "significant" results in their investigation into Anglo Irish Bank.
Speaking about the impact of the Love/Hate series on RTÉ, he said its merits one way or the other could be debated for a long time.
He noted that real life dramas are always played out in the media.