The lyrics refer to the suppression of a rebellion of Irish nationalists which began on May 24, 1798. The United Kingdom was at war with revolutionary France at the time, and the Irish nationalists were encouraged by rumours that France would invade the island. "Croppies" meant people with closely cropped hair, a fashion associated with the anti-aristocrat (and therefore, anti-wig) French revolutionaries.
The song is famous, or notorious, for being sung by members of the Orange Order in their parades, which have been made increasingly controversial since the Troubles in Northern Ireland.
The lyrics to the song begin: