Reegan was a criminal hired by General Carrington to kidnap three aliens and use them to commit acts of terror as part of his plan to start a war with their race. Later, Reegan hoped to use the aliens to commit crimes for him, but he was eventually arrested by UNIT. His last act was to suggest a way UNIT could stop Carrington, hoping to get a lighter sentence for it. (TV: The Ambassadors of Death)
The production text on the Ambassadors of Death DVD revealed that Reegan and his gang were originally meant to be Irish, and suggests it was changed after the Troubles started: "all in all, it might not have been the best moment to show Irish hoodlums planning to deploy a powerful new weapon."
In the novelisation, Reegan is portrayed as being an Irish mercenary as per the production team's original intent. His background is further explored: it's revealed that he was a former agent of the IRA and had spent time in America robbing banks to fund the organisation. He was wanted by the IRA after they learned that he wasn't giving them as much of a cut as he was keeping for himself.