The Brjáns Saga (also known as Brjánssaga) is a hypothetical early literary specimen in ancient Norse; according to this hypothesis certain episodes of the saga of Njál and Þorsteins saga Síðu-Hallssonar used as source this lost saga.

There is no evidence of the existence of the saga, although the Icelandic researcher Einar Olafur Sveinsson postulated about it when he edited the saga of Njál in 1954. Einar was convinced that the work was written around 1190 and used by the author saga Orkneyinga, written around 1200. A controversial approach by the Irish scholar Donnchadh Ó Corráin had some popular resonance for his claim about the composition of the work towards the year 1100 when Magnus III of Norway threatened Irish interests.It believed that the saga was written in Dublin in response to Cogad Gáedel re Gallaib as a way of the hibern-nordic dublinenes claiming their loyalty as descendants of Brian Ború. He used a in the "Þorsteins saga Síðu-Hallssonar" as evidence of its existence: The jarl thanked him for his words. He then traveled to Ireland and fought King Brian, and there were many remarkable events that happened at that time as mentioned in his saga.

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