The Dál Fiatach are claimed as being descended from Fiatach Finn mac Dáire, a legendary King of Ulaid and High King of Ireland, and are thought to be related to both the Voluntii and Darini of Ptolemy's Geographia. They are also perhaps more directly related to the pre-historic Dáirine, and the later Corcu Loígde of Munster. Kinship with the Osraige is also supported, and more distantly with the Dál Riata.
The Ulaid, of which the Dál Fiatach at times were the ruling dynasty, are further associated with the so-called Érainn. The Dál Fiatach claimed kinship with the legendary Cú Roí mac Dáire and the Clanna Dedad.
The Dál Fiatach are considered by scholars to be the true historical Ulaid (< *Uluti), but after the fortunes of the dynasty declined in the 7th century, the legendary heroes of the Ulster Cycle were in fact claimed as ancestors by the rival and unrelated Dál nAraidi or Cruthin, claiming for political reasons to be the "true Ulaid" themselves and descendants of Rudraige mac Sithrigi through Conall Cernach. The legendary Ulaid, a people presumably related in some way to the ancestors of the Dál Fiatach, although this is not clearly preserved in the later genealogical traditions, are sometimes called the Clanna Rudraige. However, rather than contesting the quite false claims of the Cruthin to their ancient glory, the Dál Fiatach appear to have chosen to stress their kinship with the Clanna Dedad of Munster, fearsome rivals of the Clanna Rudraige. Thus with their own ancestors appropriated by the Dál nAraidi, the Dál Fiatach apparently had no choice but to transform themselves into descendants of their nearest kin they could remember. While kinship with the Dáirine and/or Clanna Dedad (Érainn) is not contested by scholars, it can be assumed the early generations of the Dál Fiatach pedigree are quite corrupt. This is also true for the pedigree of the Dáirine and Corcu Loígde. Their natural kinship with the Munster dynasties can only be reconstructed in studies of Ptolemy's Ireland and by linguistics.
Every known king of Dál Fiatach became King of Ulster (Ulaid), but they did not monopolise the kingship as the Dál nAraidi supplied a number of powerful kings. Among the more influential Dál Fiatach kings were:
A junior branch of the Dál Fiatach ruled Lecale, the peninsula south of the Dál Fiatach capital, Dún Lethglaise (modern-day Downpatrick). Dún Lethglaise itself, already the royal centre of the Dál Fiatach would become a prestigious monastic site. In later times, from the 9th century, Bangor, originally controlled by the neighbouring Dál nAraidi, became the main religious site patronised by the kings.