Gráinne was a legendary daughter of the equally legendary pre-Christian king of Tara, Cormac (qv) son of Art. The story of her love for the tragic hero Diarmait grandson of Duibne is one of the most famous tales of the Fenian Cycle, a tale echoed in folklore, through placenames, and in the image of the romantic Celt framed by Irish cultural revivalists such as W. B. Yeats (qv). Despite its seeming antiquity, the earliest surviving literary account, ‘Tóruigheacht Dhiarmada agus Ghráinne’ (‘The pursuit of Diarmaid and Gráinne’), dates from the early modern period. However, this relatively late narrative has deep roots. Knowledge of the elopement of Diarmait with Gráinne formed part of the trained poet's repertoire by the tenth century. Furthermore, Gráinne's complicated sexual relationships are referred to in a number of texts, including the centrepiece of the Cycle, ‘Acallam na Senórach’.
‘Tóruigheacht’ tells how Gráinne is betrothed by her father to Finn (qv) son of Cumall, leader of the warrior Fian. Unfortunately, Finn is elderly and the princess falls in love with one of Finn's men, the handsome young warrior Diarmait grandson of Duibne. ‘Tóruigheacht’ introduces a classic love triangle, that between an old jealous man, a beautiful woman, and a young virile warrior duty-bound to the older man. Desperate to have her way, the sexually predatory Gráinne places Diarmait under an obligation, or geis, to elope with her, betraying his loyalty to Finn. Essentially, she shames Diarmait into becoming her lover, forcing him into exile with her. The pair flee across Ireland, pursued by an angry Finn. Gráinne is portrayed as a fickle woman whose acts endanger Diarmait. In contrast, the narrative emphasises the loyalty between Diarmait and his fellow Fian warriors, particularly Oisín (qv), Oscar, and Caílte. The hero even has a supernatural protector in the shape of the otherworldly Óengus. Eventually, Finn is persuaded to forgive the lovers, enabling them to settle down and have a family. But Finn is merely waiting: many years later he tricks his rival into hunting the boar of Benn Gulban, knowing that this will result in the breaking of geis and disaster for Diarmait. Thus, even though Diarmait kills the boar he is fatally gored. At this point Finn refuses to use his supernatural healing powers to save the dying hero, earning him the contempt of many among the Fian. Yet Gráinne proves more forgiving and accepts Finn as her husband, despite his betrayal of Diarmait. She appears with him publicly before the Fian, validating their relationship, not long after Diarmait's death. The warriors respond by shouting at her in mockery. For them it is Gráinne, even more than Finn, who is the villain of the piece. ‘Tóruigeacht’ gives an orderly, although episodic, form to a complex literary tradition. It is worth noting that this tradition also remembers Gráinne's sister Ailbe as a wife of Finn. The narrative tales portray the relationships between the kings of Tara and the Fian as being at once both mutually supportive and destructive. This duality is played out in Gráinne's own sexual preferences. She accepts, rejects, and then again accepts Finn, just as Finn is accepted and rejected by successive kings of Tara. Diarmait is a victim of these shifting allegiances.
Moreover, the tale resonated with oral tradition and folklore. Across Ireland, stone-age sites were frequently associated with the lovers’ flight, often known by names such as lebaid Dhiarmada uí Dhuibhne agus Gráínne ‘the bed of Diarmait grandson of Duibne and Gráinne’. The romantic nature of the relationship was highlighted in the anglophone reception of the tale, and the tragedy of Diarmait, in particular, resonated with Irish literary revivalists. Augusta Gregory (qv) eccentrically retold the story in Gods and fighting men, and it was the subject of Yeats's stillborn play ‘Diarmuid and Grania’. More recently it inspired the Irish dance show ‘Dancing on dangerous ground’, first performed in 1999. Yet Gráinne's choices have proved challenging to a modern audience. For instance, ‘Dancing on dangerous ground’ has a happy ending. Diarmait is not tricked to his death and Gráinne does not return to Finn. Ultimately, Gráinne's series of sexual betrayals, combining lust with self-interest and pragmatism, are far removed from the popular image of a tragically romantic Celtic Ireland.