Sechnassach mac Colggen (died 746/747) was a king of the Uí Cheinnselaig of South Leinster or Laigin Desgabair.
His father Colcú mac Bressail (died 722) was called king of Ard Ladrann (near Gorey in County Wexford) at his death obit in the annals in 722 which mention that he was killed.
[1] His brother had been slain at the Battle of Áth Senaig in 738 by the high king Áed Allán.
After the crushing defeat at Áth Senaig, the Uí Dúnlainge dominated the kingship of Leinster for fully three centuries.
[2] His nephew Eterscél mac Áeda (died 778) was also a King of the Uí Cheinnselaig.