Historians have questioned the circumstances around his death, with varying hypotheses including weakness from dysentery, intoxication, suicide, and murder.

[13] After six years, Meagher left Ireland for the first time,[16] to study in Lancashire, England, at Stonyhurst College, also a Jesuit institution.

"[15] While Meagher was at Stonyhurst, his English professors struggled to overcome his "horrible Irish brogue"; he acquired an Anglo-Irish upper-class accent that in turn grated on the ears of some of his countrymen.

[18] In June 1846, the administration of Sir Robert Peel's Tory Ministry fell, and the Liberals under Lord John Russell came to power.

Those who hoped to gain by government positions, also called The "Tail", and described as the "corrupt gang of politicians who fawned on O'Connell" wanted to drive the genuinely ecumenical Young Irelanders from the Repeal Association.

[25] On 13 July, O'Connell's followers introduced resolutions to declare that under no circumstances was a nation justified in asserting its liberties by force of arms.

William Smith O'Brien protested against this attempt to suppress legitimate speech and left the meeting with other prominent Young Irelanders in defiance, never to return.

[26][29] In January 1847, Meagher, together with John Mitchel, William Smith O'Brien, and Thomas Devin Reilly formed a new repeal body, the Irish Confederation.

The design used in 1848 was similar to the present flag, except that orange was placed next to the staff, and the red hand of Ulster decorated the white field.

[31] Following the incident known as the Young Irelander Rebellion of 1848 or "Battle of Ballingarry" in August 1848, Meagher, Terence MacManus, O'Brien, and Patrick O'Donoghue were arrested, tried and convicted for sedition.

But, due to public outcry[33] and international pressure,[34] royal clemency commuted the death sentences to Penal exile for life to "the other side of the world".

[35][36] On July 20, the day after being notified of his exile to Van Diemen's Land, Meagher announced that he wished henceforth to be known as Thomas Francis O'Meagher.

Meagher accepted the "ticket-of-leave" in Van Diemen's Land (Tasmania), giving his word not to attempt to escape without first notifying the authorities, in return for comparative liberty on the island.

A further stipulation was that each of the Irish "gentleman" convicts was sent to reside in separate districts: Meagher to Campbell Town and shortly after to Ross (where his cottages still stand); MacManus to Launceston and later near New Norfolk; Kevin O'Doherty to Oatlands; John Mitchel and John Martin to Bothwell; and O'Brien (who initially refused a ticket-of-leave) to the "Penal Station" on Maria Island and later to New Norfolk.

During his time in Van Diemen's Land, Meagher managed to meet clandestinely with his fellow Irish rebels, especially at Interlaken on Lake Sorell.

Soon after they were married, Catherine became ill.[41] Less than a year after his wedding in January 1852, Meagher abruptly surrendered his "ticket-of-leave" and planned his escape to the United States.

[42] Prior to the outbreak of the American Civil War, Meagher traveled to Costa Rica, in part to determine whether Central America would be suitable for Irish immigration.

One of his ads in the New York Daily Tribune read: "One hundred young Irishman—healthy, intelligent and active—wanted at once to form a Company under command of Thomas Francis Meagher.

[48] Under the command of Colonel Michael Corcoran, another leading Irish political figure, the 69th fought in the First Battle of Bull Run, a Confederate victory.

[53] In lectures, including a famous speech made at the Boston Music Hall in September 1861, he implored the Irish of the North to defend the Union.

This reputation was solidified when the New York printmaker Currier and Ives published a lithograph depicting Meagher on horseback, leading his brigade in a bayonet charge.

Meagher's brigade led an attack at Antietam on September 17 against the Sunken Road (later referred to as "Bloody Lane") and lost 540 men to heavy volleys before being ordered to withdraw.

[65] Meagher took no direct part in this battle, remaining at the rear when his brigade began their advance, due to, what he described in his official report as 'a most painful ulcer in the knee joint'.

[citation needed] In 1867, Montana pioneer John Bozeman was allegedly killed by a band of Blackfeet, who attacked other settlers as well.

He secured funding from the federal government to campaign against the Native Americans, but was unable to find the offenders, or retain the militia's cohesion.

[73] On the way to Fort Benton, the Missouri River terminus for steamboat travel, Meagher fell ill and stopped for six days to recuperate.

When he reached Fort Benton, he was reportedly still ill.[74] Sometime in the early evening of 1 July 1867, Meagher fell overboard from the steamboat G. A. Thompson, into the Missouri River.

[78][79] In the same vein, American journalist and novelist Timothy Egan, who published a biography of Meagher in 2016, noted that his political nemesis, Wilbur Fisk Sanders, was in Fort Benton at the same time.

Egan wrote that the ship's captain state that he was sober and ill and hypothesized that Meagher may have been set up for murder by his Montana political enemies or powerful and still active vigilantes.

When the Civil War broke out, he raised Company K, Irish Zouaves, for the 69th New York State Militia Regiment, which fought at First Bull Run under Colonel Michael Corcoran.