When the Irish and Scottish immigrated to this country following the great potato famine, they brought many of their traditions with them. Work for these immigrants was often very difficult to find. Factories and shops displayed signs reading "NINA" meaning No Irish Need Apply. The only jobs they could get were the civil service jobs that were dirty, dangerous or both -- firefighters and police officers -- jobs that no one else wanted.
Irish-American firefighters began affixing images of the shamrock to their apparatus and their person not only as a display of Irish-American pride, but also as an inconspicuous message to their fellow Irishmen advertising that the fire service is a place that can't discriminate against them.
Today, by tradition, most truck companies have a shamrock somewhere in their logo, on their apparatus, or on their helmet. Irish-American firefighters usually display a shamrock somewhere on their gear, as well, to channel the "luck of the Irish".
