Gráinne is the daughter of Cormac mac Airt. Grainne was arranged to be the bride of Fionn mac Cumhaill but Gráinne, having fallen in love with Diarmuid Ua Duibhne at her wedding party, placed a geis upon him to run away with her. The tragic love story of Diarmuid and Grainne later became the story model of Guinevere and Lancelot.
Gráinne was a beautiful woman with brown hair that reached her mid-back. She had dark colored eyes and wore a plain white dress. Diarmuid noted that her eyes were similar to Sola-Ui's, especially when the two women were pleading to him for something.
In her brief appearance, Gráinne was shown to be deeply unhappy about her marriage to Fionn. She became fixated on the idea of running away with Diarmuid from the moment she first laid eyes on him and expressed joy once Fionn forgave the couple for their actions.
Gráinne appears in a flashback that details her abandonment of Fionn for Diarmuid. Kayneth, being the Master of Diarmuid, witnesses these events through a dream. Kayneth seems to interpret the events as Diarmuid happily stealing his Lord's wife, and accuses Diarmuid of doing the exact same thing with Sola-Ui.
In Sensha Otoko, she is one of the local stalkers in the neighborhood. She is seen stalking El-Melloi girl's older brother.
Gráinne was capable of using the Geis curse, which she used on Diarmuid.
 Fate/Zero material - Encyclopedia: Diarmuid Ua Duibhne [Servant], p.100
Diarmuid Ua Duibhne [Servant]
The Lancer class Servant of the Fourth Holy Grail War, summoned by Kayneth El-Melloi Archibald. A hero of the Fianna, spoken of in Irish mythology. The story of his tragic love affair with Grainne would later become the model for the story of Lancelot and Guinevere. It's no wonder that the Berserker of the Fourth Holy Grail War has such bad compatibility with him!
In battle, he wields two Noble Phantasms: the Crimson Rose of Exorcism (Gae Dearg) which temporarily nullifies the effects of magical energy, and the Yellow Rose of Mortality (Gae Buidhe) which inflicts cursed wounds that cannot be healed by any means.
His basic parameters are not particularly high, but he excels at using battle tactics designed to trip up enemies stronger than himself. In particular, he is the natural enemy of those Heroic Spirits which rely on the powers of their Noble Phantasms to fight. Depending on his strategy, it's even possible that he could have won the entire Fourth Holy Grail War (or suffer a humiliating defeat at the hands of Assassin).
Out of all the Servants in the fourth war, he had the second-worst compatibility with his Master. Though, if his very first opponent had been anybody besides Saber, he might have been able to fight in a way more in line with his Master's expectations. For you see, it was Saber's serene beauty and grace which kindled the flames of chivalry within his heart. Also, while he was summoned as a Lancer this time, and therefore carried the twin demonic spears Gae Dearg and Gae Buidhe, his legends also speak of a pair of demonic swords: Moralltach and Beagalltach. So, there was actually a decent chance he could have been summoned as a Saber. Or rather, that's probably what Kayneth was hoping for…