The Dark Age Celts had their share of supernatural creatures within the various mythologies (Welsh/Brittany/Ireland/Scotland), in addition to the pantheon of actual gods and goddess (for Wales, see Children of Don; Children of Llyr: http://www.sarahwoodbury.com/the-sidhe/).
Here are some notable demons from Celtic mythology:
Cwn Annwn (Welsh hellhounds): Yes, they really do exist outside of the TV show, Supernatural (great show, by the way. Watch it at Netflix). The Cwn Annwn are the hunting dogs of Arawn, Lord of the Otherworld and are associated with the Welsh form of the Wild Hunt. ”The Cwn Annwn resemble small wolves. The pack leader, Fflyddmyr, is black while the other three hounds are white with red-tipped ears. Their abilities include super-speed and super-strength.” http://otherworldseries.wikia.com/wiki/C%C5%B5n_Annwn
The Fomori: ”In Celtic mythology, the Fomori are demons that live in the impenetrable darkness of the sea’s depths and in lakes and dark pools in the upper world. They were once ruled by Balor, who provided them with victims, but after his death they returned to their waters and prey on people, taking the form of sea-monsters, lake spirits, and the boggarts who lurk in the fens.” http://www.probertencyclopaedia.com/D_FOMORI.HTM
Ysbaddaden Bencawr, Chief of the Giants (Welsh): “A vicious giant residing in a nigh unreachable castle, he is the father of Olwen and uncle of Goreu fab Custennin. So huge is his frame, he requires great forks to prop up his eyelids.” http://en.wikipedia.org/wiki/Ysbaddaden
The Crow: ”The Crow features prominently within all Celtic cultures as a symbol of death. As carrion birds they act like vultures of the Celtic lands and soon arrive in droves after every battle. In the warrior culture of the Celts seeing the crows flock to a battle site and attack your fallen comrades, does much to anger a survivor. From anger, fear is just a stepping stone away. An excellent example of the use of Crows in Celtic Myth is found in the ancient Irish epic Tain Bo Cuilnge, the story of the death of Cu Chulainn. Here the goddess Morrigu attacks the hero Cu Chulainn in the form of a crow in response to his spurning of her love. In Scotland the term “Hoodie” has been applied to that of a crow and described as a half man, half crow figure who abides his presence in one form or the other depending on day or night. It is possible that this may be the early version of the Irish Banshee. In some stories of Hoodies, they are described as drinkers of Blood and may also be the Celtic version of Vampires.” http://celticevents.com/2007/03/09/roots-history-legends-the-crow-%E2%80%93-demon-of-the-celtic-world/
The Bean-Sidhe (Banshee)–Irish: ”An ancestral spirit appointed to forewarn members of certain ancient Irish families of their time of death. According to tradition, the banshee can only cry for five major Irish families: the O’Neills, the O’Briens, the O’Connors, the O’Gradys and the Kavanaghs. Intermarriage has since extended this select list.
Whatever her origins, the banshee chiefly appears in one of three guises: a young woman, a stately matron or a raddled old hag. These represent the triple aspects of the Celtic goddess of war and death, namely Badhbh, Macha and Mor-Rioghain.) She usually wears either a grey, hooded cloak or the winding sheet or grave robe of the unshriven dead. She may also appear as a washer-woman, and is seen apparently washing the blood stained clothes of those who are about to die. In this guise she is known as the bean-nighe (washing woman).” http://www.irelandseye.com/animation/explorer/banshee.html