about that name

Why does a band playing Irish music have a name like 40 Thieves?

Good question!  A little history.

The 40 Thieves is the first recorded street gang in United States' history, existing from the mid-1820s to the start of the Civil War.  Made up of Irish immigrants, The 40 Thieves operated in New York City’s Five Points District, a slum in lower Manhattan that at that time had the largest Irish community outside of Dublin.  The gang started in Rosanna Peer’s green-grocery speakeasy, which sold greens in front and cheap booze in the back.  Rosanna’s backroom became the gang’s meeting-place and headquarters.

The Forty Little Thieves were their juvenile subgroup from which they recruited.

Other, later Irish gangs also had colorful names, as well.  Among these were the Kerryonians, Roach Guards, Dead Rabbits, Chickesters, Plug Uglies, and the Shirt Tails. 

The Kerryonians, which also were formed at Rosanna’s shop, was made up exclusively of County Kerry natives. 

The Shirt Tails were so called because they wore their shirts on the outside of their trousers, like Chinamen, and the expressive appellation of the Plug Uglies came from their enormous plug hats, which they stuffed with wool and leather and drew down over their ears to serve as helmets when they went into battle. The Plug Uglies were for the most part gigantic Irishmen, and included in their membership some of the toughest characters of the Five Points. 

The Dead Rabbits were originally part of the Roach Guards, organized to honor the name of a Five Points liquor seller. But internal dissension developed, and at one of the gang's stormy meetings someone threw a dead rabbit into the center of the room. One of the squabbling factions accepted it as an omen and its members withdrew, forming an independent gang and calling themselves the Dead Rabbits (in the slang of the period a rabbit was a rowdy, and a dead rabbit was a very rowdy, athletic fellow.)

The source for much of information is the book, Gangs of New York: An Informal History of the Underworld by Herbert Ashbury  Luis Jorge Borges  Foreword by Jorge Luis Borges