Police in Northern Ireland have arrested two men in connection with a car bomb that went off in Londonderry over the weekend. br Officials described the incident as a reckless attack, which occurred outside a courthouse in the heart of the city.br Police believe the New IRA - a dissident republican group - may have been behind the attack.br So-called "dissident" republicans advocate the use of violence to achieve their aim of a united Ireland. br The New IRA is believed to have been responsible for a number of attacks in Londonderry in recent years.