He was named wool draper and mercer to King George III in 1794, an honorary title he probably owed to his friend Francis Higgins, a notorious scoundrel known as the "Sham Squire", owner of a well-known government "print" (newspaper), The Freeman's Journal.
During April–May 1798 he informed the British government several times of the whereabouts of Lord Edward Fitzgerald on Thomas Street, just as the latter prepared to take the field at the head of thousands of Croppies during the 1798 Rising.
Lord Edward's arrest on 19 May deprived the United Irishmen of their most charismatic leader, on whose head the British had put a price of £1,000, equivalent to £1.5m in 2015.
Magan sold this information on to Dublin Castle the next day, provoking Major Sirr with a body of soldiers to apprehend Lord Edward as he departed from the rear of 20 Usher's Island at dusk.
Given further information by Magan, Sirr found Lord Edward suffering from fever in 153 Thomas Street and shot him during a struggle, and he died in Newgate Prison, Dublin, some days later.