The worst in the Celtic music realm far surpasses (or sub-passes) Andy M. Stewart, I'm afraid. My favorite bad album cover was by Lyonesse, a French-based Celtic band with Breton, French, Italian and Cornish members (Trevor Crozier was in the group). Their first album was the same turquoise color as el rayo-x, but featured a blurry, hideous black and white photo that obscured every band member's face. Mireille Ben, who was the lead singer, is sitting in the back, and the guitar's headstock is right in front of her face. I can't find a jpg of the whole cover, but the photo is here. It's the first image down.