The Roman Emperor Constantine the Great converted to Christianity after a vision before the famous Battle of the Milvian Bridge, having seen a chi-rho in the sky, and thence the motto In Hoc Signo Vinces, telling him he would be victorious with the sign of the cross.

Legend has it that St. Patrick struck the shield of Conall, son of King Niall of the Nine Hostages, with his crosier, called Bachal Isu (the staff of Jesus) inscribing thereon a sign of the cross and told him the same, and baptized him.

According to the Life and Acts of Saint Patrick (chapter 138), commissioned by Sir John de Courcy and written by Jocelyn of Furness (c. 1185 AD), St. Patrick took his staff, known as the staff of Jesus, or Bacall Iosa, and struck the shield of Prince Conall, rendering a sign of the Cross on it, “et mox cum baculo suo, qui baculus Jesu dicebatur Crucis signum ejus scuto impressit, asserens neminem de stirpe ejus in bello vincendum qui signum illud”, and thus indicating that he and his offspring would henceforth be victorious in battle if they followed that sign[4] This legend is also described several centuries later in the Lebhar Inghine i Dhomhnaill.

Emperors and other monarchs, having paid respects to the Pope, descended the Scala Regia, and would observe the light shining down through the window, with the motto, reminiscent of Constantine's vision, and be reminded to follow the Cross.

In an earlier version (before Bernini's renovations in the mid-17th century), something similar may have resonated with and been observed by Prince Rory O'Donnell, 1st Earl of Tyrconnell following his visit to Pope Paul V (at the Palazzo Quirinale) in Rome, just prior to his death in 1608.

The O’Donnell rulers of Tyrconnell are also noted for having in the late 12th century given sanctuary to the Donlevy dynasty of Ulaid (Ulster), after their kingdom had fallen to John de Courcy in 1177.

It is in Tyrconnell that a branch of the Donlevy's became known as the MacNulty's, deriving from the Irish Mac an Ultaigh, meaning "son of the Ulsterman", in reference to their former kingdom of Ulaid.

[15] In the absence of these indulgences of the O’Donnell dynasty kings having maintained the MacDonlevy and MacNulty physicians as a dignified community, it is debatable whether they could have so influenced the course of western medicine, educating and training Niall Ó Glacáin (L. Nellanus Glacanus) in the medical arts, so he could later on the Continent apply empirical method to pioneer the field of forensic anatomy and pathology, first describe the petechial haemorrhages of the lung and swelling of the spleen incident of bubonic plague (Tractatus de Peste, 1629), and early elucidate the empirical method of differential diagnosis for the continental European medical community, and producing the medieval physician and medical scholar Cormac MacDonlevy translator from Latin to vernacular of Bernard de Gordon's Lilium Medicine, Gaulteris Agilon's De dosibus and Gui de Chuliac's Chirurgia.

The Ó Cléirigh were too a learned Irish royal family that had lost their sub-kingdom in Uí Fiachrach Aidhne in what is today County Galway to the Anglo-Norman forces of Henry Plantagenet.

The Ó Cléirigh then went into service of the O’Donnell as poet historians, scribes and secretaries or official bards, called in Irish language "ollam righ".

Onara bore for Donnchadh a son Mícheál Ó Cléirigh (c. 1590 – 1643), anglicized Michael O’Cleary, who matured to become the principal author of the Annals of the Four Masters.

Daniel served in the French army in the wars of the period, fighting against Duke of Marlborough at the battles of Oudenarde and Malplaquet at the head of an O'Donnell regiment.

Hugh Ambrose O'Donel, O.F.M., who adopted the modern version of the name 'O'Donnell', a Franciscan priest in Killiney, retired from missionary work in Zimbabwe, who died on 11 July 2023.

Thomas O'Donnell (1871–1943), MP for West Kerry from 1900 until 1918, was a leading agrarian reformer, and the first Member of Parliament to address the House of Commons in Westminster in the Irish language (Gaelic), but was called to order by the Speaker, but not without having made his mark with John Redmond's support.