The Irish surname Sheahan / Sheehan is an anglicized form of the Gaelic Ó Siodhacháin. The prefix "O" signifies "grandson of" or "descendant" and indicates the patronymic origin of the name while the first name would seem to be derived from the Irish word "siodhach," peaceful. The majority of the Sheahan's, now very numerous in counties Cork, and Kerry and Limerick, are of Dalcassian descent, that is descended from Cas, son of Olliol Ollum, first absolute king of Munster, AD 177. The principal family of the name were part of the Dál gCais, the tribal grouping occupying an area now in counties Limerick and Clare which produced Brian Ború, High King of Ireland in the eleventh century.

Some of the traditional genealogies have the descent of the Sheehan's from one Sidhechan, a contemporary of Brian Ború, and distantly related to him. Initially they appear to have lived in the south of Co. Limerick, in the barony of Connello. In very early times, however, they migrated south, into the north-east of the present Co. Cork, where they are still most numerous. Over the course of the centuries, large numbers have also migrated into Co. Kerry, while a significant number also remained in their homeland of Limerick. It is with Cork and Kerry that the name is now indelibly associated.

A separate family of Ó Siodhacháin is also recorded in medieval times among the Uí Maine in the Galway/Roscommon region. The Ui Maine sept of this name were hereditary trumpeters to the O'Kellys, but very few of the name remain in this area.

Today, Sheahan is one of Ireland's most numerous surnames combining the alternative spellings Sheehan (80%) and Sheahan (20%). It is seldom found with its prefix "O". Notable bearers of the name include Canon Patrick Augustine Sheehan (1852-1913), poet and novelist, and Michael Sheehan (1870-1945), Archbishop of Sydney, who was associated with Ring College and the revival of the Irish Language.

The Sheahan family motto is "semper paratus" (always prepared).