Geasa (singular geis) are the sacred prohibitions that define and doom Irish heroes: to violate a geis brings catastrophe, but circumstances are often engineered so that two geasa conflict, leaving no safe path. The fate of Diarmait, bound by a geis of hospitality to hunt a fatal boar, is a representative example. For poets, the geis represents the power of language — specifically the language of prohibition and obligation — to structure destiny. The figure of the hero trapped between incompatible sacred obligations creates a tragic architecture that Irish dramatists and poets have repeatedly explored as a model for the conflicts of the modern Irish condition.

Geasa are supernatural taboos or obligations placed on heroes, creating narrative tension and moral dilemmas. Breaking geasa leads to disaster, highlighting the power of fate and social contract. In poetry, geasa represent the constraints on human freedom and the importance of honoring commitments. They often involve prohibitions on food, place, or action, testing the hero's ingenuity. Geasa illustrate the intersection of the mundane and the magical in Irish worldview. They are key to understanding the tragic structure of many myths.