Roger Casement was not interested in a blood sacrifice. He believed that an Irish rebellion had every chance of success as long as there were plenty of German soldiers and guns behind it.

In 1915, he and Joseph Plunkett wrote a 32-page memorandum for the Berlin government that outlined exactly how this could be achieved - culminating in a victory parade down O'Connell Street with people throwing flowers while bands played A Nation Once Again and Deutschland Uber Alles.

Casement's dream never came to pass. Instead, he ended up as the Easter Rising leaders' odd man out.

While the other rebel martyrs were shot in the Stonebreakers' Yard of Kilmainham Jail, he never made it to Dublin and died on the gallows in London's Pentonville Prison.

Idealist

Casement was unique in other ways too. He had close links with almost all of the Rising's key supporters in Ireland, Germany and the United States. None of them fully trusted him and he remained what he always had been - a loner.

He was an idealist, a humanitarian and a man of great personal charm. He was also a manic depressive who suffered from paranoia and wild mood swings.

To make his life even more difficult, he was homosexual in an age when this had to be hidden as a shameful secret.

Casement's Anglo-Irish background made him a hard man to pin down. Born in Dublin, he split his childhood between Sussex and the Glens of Antrim.

He joined the Foreign Office and as a young man seems to have considered himself both British and Irish.

Casement first became famous as a fearless investigator of human rights abuses in the international rubber trade. He wrote sensational reports from the Belgian Congo and Peru, revealing how natives who failed to gather enough rubber had their limbs hacked off.

These crusades made him a hero and in 1911 he knelt before King George V to receive a knighthood.

Despite his new status, Casement's African and South American experiences had left him with a fierce hatred of imperialism. He became active in Ireland's nationalist movement, joining Sinn Fein and the Irish Volunteers.

He helped to finance and organise the Howth gun-running in July 1914, which delivered the arms that made the Easter Rising possible.

When the First World War broke out, Casement was in New York holding talks with the Irish republican group Clan na Gael. They gave him permission to visit Germany and drum up support for a rebellion.

In particular he wanted to recruit a military brigade made up of Irishmen who had joined the British army and ended up in German prison camps.

Casement's mission was ultimately a dismal failure. Almost all of the Irish prisoners he approached refused to switch sides, prompting him to bitterly complain that they were "cads and cowards". While the Germans were intrigued by his plans, they would only agree to send a small quantity of arms.

In April 1916, a German cargo vessel called the Aud set sail for the Kerry coast with 20,000 rifles on board. They were followed a few days later by Casement in a submarine.

Intercepted

By now he was determined to stop the Rising as he knew it had no chance of succeeding.

Sadly, everything that could go wrong did. The Aud was intercepted by British warships and forced to scuttle.

Casement and his two Irish companions managed to land at Banna Strand, but their dinghy overturned close to shore and they collapsed on the beach both soaked and exhausted.

Within a few hours, the local police had tracked Casement down in a local fort. He was arrested, escorted to England and locked up in the Tower of London - where he made two unsuccessful suicide attempts.

His subsequent trial for treason was a nasty affair. The British leaked extracts from his infamous 'Black Diaries', which showed that he regularly paid young men for sex.

While there have been persistent conspiracy theories that the diaries were forged, most historians now accept them as genuine.

Casement was found guilty and hanged on August 3. Afterwards, his naked body was thrown into an open grave. In 1965 the British government sent his remains home and he received a State funeral in Glasnevin Cemetery.

W.B. Yeats later wrote a poem with the refrain: 'The ghost of Roger Casement is beating on the door'. In the more tolerant Ireland of 2016, that ghost can finally be laid to rest.