Here is a brief history of the origin of the O'Mullally or Lally clans, provided here by John Mullaly
(firstname.lastname@example.org) from the book "History of the O'Mullally and Lally Clann" written by D. P. O'Mullally and
published in Chicago in 1941.
The O'Mullally or O'Maolalaidhs (Gaelic spelling) clann come from the kingdom of Hi-Maine. The settlement was made in the south-east of Connacht by Prince Maine Mor of Oriel. Records in possession of the O'Kelly family in that territory state that this settlement was made in the year 357A.D. This Maine Mor seemingly was the last pagan prince from whom the O'Maolalaidhs trace their descent.
Father Keating in his history of Ireland states that Hi-Maine comprised the baronies of Ballymoe, Ballintober, South Athlone, and Moycarn in County Roscommon, and also Ballymoe, Traquin, Killian, Kilconnell, Cluainmacnowen, Longford, Leitrim, Loughrea and portions of Upper Tulla in County Clare. This fair land was occupied by the Firbolgs but they were not numerous. Previous to the coming of the Hi-Manians, St. Grellan had built a church at Cill Cluane (Kilclooney). The new settlers first camped upon Maen Magh, which was to remain with the senior branch, namely, the O'Neachtains and the O'Mullally's and there they were welcomed by St. Grellan who presented them with the territory on the order of St. Patrick.
The Firbolgs, thirty thousand strong, under chief Cian having agreed to a peace with the Hi-Manians who were located at Bearnach na n-arm, later called Seisidh beag,in Maenmagh, arranged for a feast with the latter but planned treachery. And St. Grellan from his church near there saw the weapons of the Firbolgs and warned the Gaels and we read that many of the Firbolgs were lost in the bog of Magh Liach. The Hi-manians paid tribute to St. Grellan and he became their patron saint and as such he is today to the Clann Mullally.
To preserve the continuity of our pedigree we reiterate that the O'Mullally family decends from Amlaibh, the son of Fiachra Finn, who was the grandson of Maine Mor, who in turn was the gr. gr. grandson of Colla da Chrioch the grandson of the High King of Ireland.