Liam Sean O’Brien was born in 1829 Dublin to surgeon Charles O’Brien and writer Sonia, and came to age in the days of the Hungry Wolves, the Great Potato Famine. While all of Europe was affected by the infamous blight, Ireland was doubly damned by British policies at the time. While there were initially attempts to soften the blow by importing other staple crops and banning exports of the surviving crop from Ireland, Lord Russell would reverse most of those measures upon taking the office of Prime Minister in 1846. While many Irishmen would flee the famine (and British oppression), including his cousin, Liam R. O’Brien, he would remain on the island and study law and Gaelic.

An incredibly quiet and private man, the exact path of his political opinions and ambitions is unknown, distorted further by attempts to vilify him after the return of Fionn mac Cumhaill (also known as the Thumbsucker Giant). While studying law, he wrote an essay favorable to Repeal campaigner Daniel O’Connell, though the focus of the essay was less on his domestic politics and more on their intersection with his opposition to slavery in the US. While rumors of his participation in the Young Irelander Rebellion remain unsubstantiated, several articles in The Nation, an Irish nationalist newspaper, are now belived to have been written by him under an assumed name. Letters bearing his writing style have been found in the effects of members of the Fenian Brotherhood and the Irish Republican Brotherhood, but said letters imply only friendship and some scant financial support. Among his friends, he was considered to be overwhelmingly mild, favoring a cautious approach towards most endeavors, and emotionally withdrawn.

It was a surprise to those who knew him when he emerged as a key figure of the New Fianna Brotherhood, known for assassinating multiple figures in British government between 1875-77. While he was not involved in the violent parts of the movement, he did serve as translator for the group’s leader, Fionn mac Cumhaill, who only spoke and wrote in Gaelic. He also wrote consistently in support of Fionn and against England during this time, insisting that United Kingdom of Great Britain and Ireland would always default to the needs of the British elite over Irish concerns and that any attempt at compromise would inevitably be overrun. He also spoke against the British empire as a whole, decrying the cruelties inflicted in the name of preserving British control over the world. While Fionn had significant support among Ireland’s less wealthy, support for O’Brien would be more mixed due to his stances on religious freedom (namely the coexistence of Irish Catholics and Irish Protestants) and support for Jewish emancipation. Outside of Ireland, when he wasn’t denounced as a traitor to Britain, he had a reputation of being laser-focused on Irish interests and the UK, neglecting attempts by reformers elsewhere to get his support. He would also be criticized in the years after his death for not pushing back against Fionn’s misogynist comments about his ex-fiancée, Gráinne.

O’Brien spent the rest of his life in hiding, as the UK condemned the NFB as a treasonous organization and had any mortal adherents to the cause arrested or killed as their mythological experts looked for a way to defeat Fionn, largely centered around finding and reviving King Arthur as a counter. Despite his best efforts, he was assassinated in 1885 while recuperating from a broken ankle, stabbed through the heart. The killer was suspected to be Irishman Henry Dillon, whose father was assassinated by the NFB in 1876 and was visiting the relevant area at the time, but he was acquitted by the courts. Driven by the death of the man who helped revived him and gave him purpose, Fionn resumed attacks on government officials. In a year, he took Dublin Castle and forced the UK to allow Ireland to secede. His reign as king would only last a few months before he abdicated and returned Ireland to democratic rule, claiming that this was no age for the rule of greater beings.

Fionn has largely returned to slumber and seclusion, awaking every few decades to speak his mind on the current state of Irish politics and make any displeasure known by the sword. Liam O’Brien is remembered nationally as a hero, though his reputation remains mixed in the rest of Europe; the great-great-grandson of his cousin, John K. O’Brien, would return to Ireland to commemorate the third statue built in Dublin in his memory. The connection between Victor Freestone and Fionn’s revival was only discovered in 2024, as copies of telegrams sent by Dr. Theodore Birch were found in an attic in Utica, NY; while this has not damaged his reputation significantly in Ireland, it has led to increased foreign pushback against Irish self-rule.