One of the most feared weapons of the fairies was the fairy stroke or poc sí, sometimes also called the fairy blast. There are several modern Irish expressions associated with this term including 'poc aosán' which is a term for a sudden illness, 'poc mearaidh' meaning a touch of madness, and 'buaileadh poc air' meaning to be elfstruck or bewitched (O Donaill, 1977). In Old Irish this might be called poc aosáin [fairy stroke] or áesán [fairy sickness]. Associated with the Slua Sí [fairy host] and the sí gaoithe [fairy wind] the fairy stroke was a sudden and otherwise inexplicable illness marked by a change in behavior and health. MacKillop suggests that this term is where we get the term stroke from for cerebral hemorrhages or aneurysms (MacKillop, 1996).
The fairy stroke could afflict both humans and animals but was differentiated from the similar elfshot in its symptoms and method of application. Unlike elfshot which used an arrowhead, sometimes invisible, to injury a person, fairy stroke was caused by a blow from the fairies themselves, or in rare cases being struck by a blunt object they threw. Fairy stroke might manifest as a sudden seizure or else a loss of mental acuity, which may be temporary or permanent (MacKillop, 1996). Getting the fairy stroke, like many things associated with fairies could be a double edged blade as it cost a person their health and mind but was also believed to convey a special esoteric knowledge (Wedin, 1998). There was also some crossover with changeling folklore as in some cases those who had received the fairy stroke were said to have actually been taken by the fairies while either a glamoured object or decrepit fairy was left behind instead (MacKillop, 1996). This is also true of those afflicted by elfshot indicating that both could be used either to torment people or as a means of taking those humans who the fairies desired.
Those who were struck by the blast might simply be at the wrong place at the wrong time, may have transgressed a fairy rule, or may have failed to adequately protect themselves. One anecdote from Newfoundland tells of a woman struck by the fairy blast because she passed through a crossroads without carrying a bit of protective bread in her pocket while another man received the blast for trying to cut down a tree the fairies didn't want cut (Reiti, 1991). In other examples people were approached by fairies who either offered them items or wanted them to do things and when the people refused the Fey folk threw items at them; wherever the item struck the person was afflicted with pain, sometimes resulting in lifelong debility and other times in madness and eventual death (Reiti, 1991).
Lady Wilde includes this charm for curing the fairy stroke in her book:
"There is a very ancient and potent charm which may be tried with great effect in case of a suspected fairy-stroke.
Place three rows of salt on a table in three lines, three equal measures to each row. The person performing the spell then encloses the rows of salt with his arm, leaning his head down over them, while he repeats the Lord's Prayer three times over each row--that is, nine times in all. Then he takes the hand of the one who has been fairy-struck, and says over it, "By the power of the Father, and of the Son, and of the Holy Spirit, let this disease depart, and the spell of the evil spirits be broken! I adjure, I command you to leave this man [naming him]. In the name of God I pray; in the name of Christ I adjure; in the name of the Spirit of God I command and compel you to go back and leave this man free! Amen! Amen! Amen!" (Wilde, 1888).
MacKillop, J., (1996) Dictionary of Celtic Mythology
Wilde (1888) Ancient Legends, Mystic Charms, and Superstitions of Ireland
Wedin, W., (1998) The Sidhe, the Tuatha de Danaan, and the Fairies in Yeats's Early Works; William Butler Yeats Seminar
Reiti, B., (1991) 'The Blast' in Newfoundland Fairy Tradition
O Donaill, (1977) Foclóir Gaeilge-Béarla