A Breton work of some controversey, published in 1839. The young Vicomte Théodore Hersart de la Villemarqué claimed that it was a collection of ancient Breton songs, compiled from the countryside, as well as from songs sung to him by his nurse. The book was a major success, acting as a literary jump-start to the Breton nationalist movement, and reflecting the wider growth in Celtic nationalism that had begun in the late 18th century. It went through three editions, the first in 1839, the second in 1845, and the third in 1867.
The songs themselves are a mixture of the typical ballad sort--fairy queens, dying knights, and even a mention of druids, Merlin, and the type of sword dance found in May Day celebrations (not unlike that seen in the film The Wicker Man).
By 1869, it was believed that these songs were forgeries. This would not be unprecedented, making Villemarqué the Breton equivalent of James Macpherson and Iolo Morgannwg. However, even this is unfair to all three men--for in each case, they produced not a complete forgery, nor the authentic ancient poetry each man claimed it to be, but something which lies in between.
In the case of Barzaz Breiz, Villemarqué is believed to have actually collected some oral literature, as demonstraited by Donatien Laurent's 1989 work Aux Sources du Barzaz-Breiz. Laurent reportedly found some of the original notebooks that Villemarqué used when interviewing people, thus showing that he did not invent the songs of wholecloth. Instead, not unlike Iolo Morgannwg, he re-edited them, embellishing and mystifying them where he thought appropriate. In this, he was not unusual for the time.
A few examples of translated works from the Barzaz Breiz can be found in the book Lyra Celtica.
For your further exploration:
Barzaz Breiz (in Breton and French): http://perso.wanadoo.fr/per.kentel/index_barzaz_breiz.htm
Lyra Celtica: http://www.sundown.pair.com/Sharp/Lyra%20Celtica/celtica_contents.htm
Laurent, Donatien. Aux Sources du Barzaz-Breiz : la mémoire d'un peuple. Douarnenez: ArMen, 1989.
Bempéchat, Paul-André. "Allons enfants de *quelle* patrie? Breton Nationalism and the French Impressionist Aesthetic." Center for European Studies Working Paper Series. #106. URL: http://www.ces.fas.harvard.edu/publications/bempechat.pdf