O'Halloran families may spring from two separate origins in Ireland. The first was of County Galway and they served as chiefs of Clann Fearghaile, a territory near Lough Corrib.

These O'Hallorans are given by O'Flahertys Ogygia, on the east side of the river Galway, near Lough Corrib. From this line was descended O'Halloran the historian. According to O'Hart, Clan Fergail territory included Galway city anciently, and the family held a castle at Barna, near the sea 3 miles west of Galway. They held 24 townlands east of the river "Gallimh" including, Roscam, about 3 miles S.E. of Galway. In the 13th century, along with Flahertys, there were driven into west Connaught where they built the castle of O'Hery in Gnomore, and according to tradition the castle of Rinvile, in northern Connemara. According to Hardiman, another O'Halloran family, of Munster, was also present in Thomand.

The Thomand family is found fairly close to the above O'Halloran family in Clare, in County Clare, near Lough Derg. This family has also branched out into Limerick.

Due to the close proximity of these two seperate families, specific origins in bordering areas may be difficult to figure out. Branches of both families are sure to be found in Limerick.

At the end of the last century (1800) O'Halloran was centered only in Limerick, and Halloran was found in Clare, Galway and Cork according to the birth index.

In 1659 Hallurane and O'Hallurane were the principle names of County Clare.

The Galway O'Halloran sept had their territory near Lough Corrib and retained their leading position in Iar-Connacht up until the end of the sixteenth century. The Clare sept were located in Ogonneloe, beside Lough Derg and here of the same origin as the MacNamaras of Thomond. This sept produced most of the notable bearers of the name. Sylvester O'Halloran, 1727-1807, was a surgeon, historian, antiquary and Irish language enthusiast and his borther Rev. Joseph Ignatius O'Halloran, S.J. 1718-1800, was professor of Philosophy at Bordeaux. The descendants of both septs are today numerous in Counties Clare and Galway.