In Irish, a geas (alternatives: geis, géis, deas; plural geasa) is an idiosyncratic taboo, whether of obligation or prohibition, similar to being under a vow. The plural geasa is also used to mean specifically a spell prohibiting some action, common in Irish folklore and mythology. It is this additional meaning of the plural which the article discusses.
A geas can be compared with a curse or, paradoxically, a gift. If someone under a geas violates the associated taboo, the infractor will suffer dishonor or even death. On the other hand, the observing of one's geas is believed to bring power. Often it is women who place geasa upon men. In some cases the woman turns out to be a goddess or other sovereignty figure.
The geas is often a key device in hero tales, such as that of Cúchulainn in Irish mythology. Traditionally, the doom of heroes comes about due to their violation of their geas, either by accident, or by having multiple geasa and then being placed in a position where they have no option but to violate one geas in order to maintain another. For instance, Cúchulainn has a geas to never eat dog meat, and he is also bound by a geas to eat any food offered to him by a woman. When a hag offers him dog meat, he has no way to emerge from the situation unscathed; this leads to his death.
In some cases the placing of a geas can lead to tragedy even when it is not violated. Aoife imposed three geasa on Connla, her son with Cú Chulainn: He cannot turn back once he starts his journey; he must not refuse a challenge; and he must never tell anyone his name. She then sent Connla, aged seven, to seek out his father -- but he was a child of such extraordinary skill that he was seen as a threat after having defeated all Ulster heros who met him. Because of the geas placed on him by his mother, he refuses to identify himself which leads to his own father, Cú Chulainn, killing him in single combat using the Gáe Bulg before recognising too late who he was. He then introduces his dying son to the men of Ulster as a fitting hero.
A geas might appear beneficial by involving a prophecy that a person would die in a particular way so bizarre that they could then avoid their fate for many years. However as with Conaire Mór, in the tale of Togail Bruidne Dá Derga, who strictly observed a number of geasa, a small unconnected infraction can escalate to one's undoing. By initially making exceptions to crimes of stealing by his foster-brothers contravening fír flathemon, the king's upholding of true judgement, things proceed until they deliberately contravene a geis of Conaire's against marauding in his reign. Though he tries to rectify the situation by exiling them his fate intervenes so the remaining geasa are involuntarily and accidentally broken one after the other with a sense of gathering doom which cannot be checked.
In the Irish saga of Conchobar mac Nessa, the king is said to have the right to the first night with any marriageable woman and the right to sleep with the wife of anyone who hosted him. This is called the Geis of the king. Whether this right actually existed and was exercised by the Celts is not attested outside the sagas. It is similar to the Droit du seigneur of feudal Europe.
For example, the Welsh hero Lleu Llaw Gyffes (in one version of his story) was destined to die neither "during the day or night, nor indoors or outdoors, neither riding nor walking, not clothed and not naked, nor by any weapon lawfully made." He was safe until his wife, Blodeuwedd, learning of these foretold conditions, convinced him to show her how he could theoretically be stepping out of a river onto a riverbank sheltered by a roof and put one foot on a goat, and so on, thus enabling the conditions that allowed him to be wounded.
Prohibitions and taboos similar to geasa are also found in more recent English literature, though they are not described as geasa in those texts. For example, in William Shakespeare's play Macbeth, the title character believes he is safe because "no man of woman born shall harm Macbeth". However, his nemesis Macduff was "from his mother's womb untimely ripp'd" (i.e., born by Caesarean section), and was therefore not "of woman born".
Another example is the Witch-King of Angmar from Tolkien's Legendarium, who has a geas-like prophecy described by the Elven hero, Glorfindel: "Far off yet is his doom, and not by the hand of man shall he fall." In this the meaning is quite literal, for the Witch-king eventually falls at the hands of Eowyn and Meriadoc, one a shieldmaiden of Rohan, and thus not a man but a woman, and the other a hobbit, and thus not a Man as in species.