What is a Wren boy?

The wren, the wren, the king of all birds,
St. Stephen's Day was caught in the furze;
Though his body is small, his family is great,
So, if you please, your honour, give us a treat.

The new year is represented by a robin, and the old year is a wren.

By National Library of Ireland on The Commons - December 26Uploaded by oaktree_b, Public Domain, https://commons.wikimedia.org/w/index.php?curid=17816307

I'm talking about an old Celtic tradition involving a procession through the streets with a dead wren. This link explains everything: http://www.from-ireland.net/custetc/wrenboys.htm