Fiachnae mac Báetáin (died 626), also called Fiachnae Lurgan or Fiachnae Find, was king of the Dál nAraidi and High King of the Ulaid in the early 7th century. He was a son of Báetán mac Echdach and brother of Fiachra Cáech (died c. 608), grandson of the Ulaid king Eochaid mac Condlai (died 552) and father of Mongán mac Fiachnai.
The Dál nAraidi kingdom was in fact a number of competing Cruthin tribes at this time so succession to the kingship was achieved through force of arms and prestige rather than by any regular means. According to the genealogies, Fiachnae's predecessor was his great-uncle Áed Dub mac Suibni, who died c. 588, and Fiachnae became king some time after Áed Dub's death. The kingship of the Ulaid was contested by Dál nAraidi and Dál Fiatach kings, so Fiachnae again did succeed directly to the kingship but required some time to impose himself as high king after the death of his predecessor, again Áed Dub. There is reference in the Annals of the Four Masters dated 597 describing the Battle of Cuil Cael where he defeats the Dal Fiatach leader Fiachnae mac Demmáin and from this could date his true overlordship of Ulaid.
While no historical sources for Fiachnae's life now remain, excepting a few bald entries in the Irish annals, a number of later traditions and a lost poem called Sluagad Fiachnae meic Báetáin co Dún nGuaire i Saxanaib (The hosting of Fiachnae mac Báetáin to Dún Guaire (Bamburgh?) in the kingdom of the Saxons) suggest that he was a significant figure in his time, campaigning against Edwin of Northumbria and perhaps also against Edwin's predecessor Æthelfrith of Northumbria. He may have captured Bamburgh - or only besieged it - circa 623.
Literary sources claim that Fiachnae's mother, who is said to have come from the Dál Fiatach, conceived him as a means of revenge against her husband. Báetán did not like the child and once set a ferocious dog on him, which Fiachnae killed by spearing it through the heart with a meat-spit.