Shane's support was considered worth gaining by the English even during the lifetime of his father Conn O'Neill, 1st Earl of Tyrone (died 1559).

[7] This was rendered as anglicisations such as "Donnolloh" in contemporary manuscripts,[8] and as "John, or Shane Doulenagh O'Neil" in Abbé MacGeoghegan's 1758 History of Ireland.

The nickname "Shane the Proud" (Irish: Seán an Díomais), which appears in nineteenth- and early-twentieth-century popular histories, was coined sometime after his death by English writers, and originally had the pejorative meaning of "arrogant", because they wished to portray him as vain, self-indulgent and ruthless, and thus undermine the legitimacy of his claim to the earldom of Tyrone.

[10] Holinshed's Chronicles of 1587, for instance, had a side-note, "The proud taunts of Shane O'neile", the text remarking that "when the commissioners were sent to intreat with him vpon sundrie points, they found him most arrogant & out of all good order, braieng out spéeches not méet nor séemelie.

However, whereas in Gaelic custom the successor to a lordship was elected from his kinsmen in the system of Tanistry, the English insisted on succession by the first-born son or primogeniture.

The case for Mathew's disqualifying status under both English and Irish law, as an affiliated member of the family rather than as an actual son of Conn Bacach,[20] was carefully stated by Shane when he made his own claim to the title of Earl of Tyrone both before and during his visit to Queen Elizabeth in 1562,[21] and restated in some detail by the English authorities when Hugh O'Neill was outlawed during the Nine Years' War.

O'Neill refused to put himself in the power of the Earl of Sussex, Thomas Radclyffe, without a guarantee for his safety; and so Elizabeth decided to establish Brian in his place.

Elizabeth, whose prudence and parsimony were averse to so formidable an undertaking as the complete subjugation of the powerful O'Neill, desired peace with him at almost any price.

Elizabeth's faith in Sussex's aggressive strategy diminished when the repeated annual devastations of O'Neill's territory by the Lord Deputy with sizeable and expensive armies failed to bring him to submission.

The O'Neill offered some concessions, most significantly consenting to present himself before Elizabeth in London to argue his case against Sussex and the Baron of Dungannon in person.

William Camden describes the wonder which his gallowglasses occasioned in the English capital, with their heads bare, their long hair falling over their shoulders and clipped short in front above the eyes, and clothed in saffron-dyed shirts of fine linen.

Elizabeth was less concerned with the respective claims of Shane O'Neill and the Baron of Dungannon, the former resting on Gaelic law, the latter on an English patent, than with the question of policy involved.

He fought an indecisive battle with Sorley Boy MacDonnell near Coleraine in 1564, and the following Easter hosted his entire army at Feadan above Newry.

Catherine was also the former wife of Archibald Campbell, 4th Earl of Argyll, whose favour could ensure Shane a ready supply of Highland "Redshank" mercenaries.

Shane kept Calvagh imprisoned at Benburb and his island stronghold of Fuath na nGall (translation: "Hatred of Foreigners") on the shore of Lough Neagh for many years.

[7] Between May and June 1567, while the O'Neill was attempting to negotiate a military alliance with the MacDonnells in the wake of his catastrophic defeat at Battle of Farsetmore, he discussed the possibility of divorcing Catherine MacLean to marry his current lover, Agnes Campbell, widow of James MacDonald; the O'Neill had captured her with her husband at the Battle of Glentasie in 1565.

[22] The O'Neill was, however, still married to Catherine on 2 June 1567, the day of his assassination at Castle Cara, Cushendun, at the hands of a MacDonnell group with whom he was negotiating possible military aid.

Catherine made her way to safety at Duart Castle, where her brother fostered the youngest of Shane's children, those who had been born to his sister, while offering protection to the other MacShanes.

Attended by a small body of gallowglass, and taking his prisoner Sorley Boy with him, he presented himself among the MacDonnells near Cushendun, on the Antrim coast, hoping to propose an alliance.

Unbeknownst to Shane, the Scots had already come to an agreement with Henry Sidney and William Piers, seneschal of Clandeboye, commander of the English garrison at Carrickfergus.

The Church of Ireland Bishop of Clogher, Miler Magrath, said "the people [of Ulster] adhere to the MacShanes, whom they consider the true branch of Conn Bacach's line", but with their arch-enemy Hugh O'Neill, 2nd Earl of Tyrone, entering into warfare with the outbreak of the Nine Years' War the MacShanes were compelled to support Hugh's enemy, the Dublin administration, and their support in Tyrone withered.

Antrim GAA has a Gaelic Athletic Association club named in his honour, Shane O'Neill's GAC, founded by the solicitor and antiquarian Francis Joseph Bigger.

A cairn was raised at his reputed burial place above Cushendun by the antiquarian Francis Joseph Bigger in 1908 and yearly commemorations were held in Shane's honour between that date and 1914.

The poet Robinson Jeffers visited the site in 1929 and refers to Shane's Cairn in several poems in the sequence Descent to the Dead, inspired by his pilgrimage to Ireland.