One of the Irish septs is a branch of the Ui Maine whose name in Irish is "O Cuagain" and are native to Galway and Roscommon, and now are found scattered through those counties and into Leinster. This surname is now found as Coogan.
Another Irish sept was Mac Cogain or "Mac Cagadhain" (also McCogane, MacCogan, MacCoggan), whose root name is from the Gaelic cogagh (aggression, strife, contention). The name originated in the 11th Century with the chiefs of Clann Fearnaighe (anglicized Glanfarne) whose territory was in Leitrim, along the shores of Lough Allen. A branch of the sept later settled in Meath. In the crest for this family, there is a Gaelic motto which translates as "The red hand of Eire." Descendants of this line are most frequently found as Cogan or Coggan (the Mac having been dropped in the 18th century) but occasionally will be seen as Coogan.
The Irish MacEochagain clan (Gaelic - "possessing horses") was centered in Cavan towards the close of the 15th Century. They spread into adjacent areas of Monaghan. This surname has sometimes been found as Coogan, although it may also be identified as Keogan.
The Norman family claims descent from kinsmen of Milo de Cogan (died ca. 1183) who was Strongbow's second in command at the Norman Invasion of 1171. He was granted lands in Cork, and the family was powerful in that area into the 1500's. Descendants of this line often use the name Cogan, Coggan, Goggan, etc., in addition to Coogan.