Táin Bó Cúailnge (Irish pronunciation: [t̪ˠaːnʲ boː ˈkuəlʲɲə]; “the driving-off of cows of Cooley”, more usually rendered The Cattle Raid of Cooley or The Táin) is a legendary tale from early Irish literature, often considered an epic, although it is written primarily in prose rather than verse. It tells of a war against Ulster by the Connacht queen Medb and her husband Ailill, who intend to steal the stud bull Donn Cuailnge, opposed only by the teenage Ulster hero Cú Chulainn. A similar tale set in the West of Ireland is Táin Bó Flidhais.
Traditionally set in the 1st century AD in an essentially pre-Christian heroic age, the Táin is the central text of a group of tales known as the Ulster Cycle. It survives in two main written versions or “recensions” in 12th century manuscripts, the first a compilation largely written in Old Irish, the second a more consistent work in Middle Irish.
You can read more about this legend at the link below:
Sources: wiki and sacredtexts .com