Not to be confused with his brother Sihtric

Sichfrith mac Ímair (Old Norse: Sigfrøðr Ívarrsson; died 888) was a ninth-century King of Dublin. He was a son of Ímar and a member of the Uí Ímair.

https://en.wikipedia.org/wiki/Sichfrith_mac_%C3%8Dmair

b) SIGFRITH (-killed 888). He presumably succeeded his brother as King [of Dublin]. The Annals of Innisfallen record in 883 "the burning of Les Mor by the son of Imar"[1175]. It is supposed that this passage refers to Sigfrith who had (presumably) succeeded his brother two years earlier. The Annals of Ulster record the death in 888 of "Sigfrith son of Imar king of the Norseman…killed by his kinsman"[1176].

http://fmg.ac/Projects/MedLands/IRELAND.htm#_Toc389126206