Not only does it appear to be authentic, it has also been revealed, after the extensive cleaning work that it has been through since its discovery, to be of high quality and although in areas the paint is flaky and the painting has lost some of its original lustre the details remain fairly clear. When it was discovered it was covered in layers of dirt, this partially explains its relegation to obscurity over the last centuries as it obscured both the intricacy of the work and of course the all important signature.
While here we see a figure suffering the effects of over-indulgence in the wine, while behind him two individuals appear to be fighting, suggesting at the various evils of drink.
Or this detail, where we see a pick pocket taking advantage of the chaotic scene.
The painting is set to go on display at the Prado tomorrow! I only help to be able to go and see it first hand one day in the not too distant future. Until then I will have to content myself with reading about it on the incredible website resource http://www.museodelprado.es/en/research/estudios/el-vino-de-la-fiesta-de-san-martin-pieter-bruegel-el-viejo/
However there are also some fantastic works by this intriguing artist in London.
For example the very different 'The Adoration of the Kings' in the National Gallery
Or the Escape into Egypt mentioned in a previous post. http://articulatelondon.blogspot.com/2011/11/ice-skating-in-style.html in the Courtauld Collection, where you could also see Christ and the Woman taken in Adultery