The past seems unalterable and unbending, though that depends on the discovery of new information and the perspective of the observer.
There is a danger of imputing more principle to historical figures than we concede to our political contemporaries, forgetting that causes serve as rallying points.
Min Ryan, later wife of Gen Richard Mulcahy, asked Tom Clarke in the GPO, who headed the signatories of the Proclamation: "Why a republic?"
He replied: "You must have something striking in order to appeal to the imagination of the world" (Annie Ryan, Witnesses Inside the Easter Rising, Liberties Press, 2005, p157).
The United Irishmen, who came before a secret committee of the Irish parliament in 1798, gave much the same answer.
Causes also need allies. Many of the United Irishmen would not rise without the French. The prospect of German aid, explicitly invoked in the Proclamation with its reference to being supported by "gallant allies in Europe", was important for strengthening the credibility of the Rising among rank-and-file volunteers even if the stark reality was self-reliance on extremely limited resources.
Min Ryan thought Eoin MacNeill's countermanding order was "the mercy of God", because "if we had a big rising, we would have had a lot of destruction".
Roger Casement's mission to Germany from the end of October 1914 to April 1916, while often deemed a failure if not a fiasco, provided a visible link to potential allies, even if, like everything he did, it had a largely freelance and quixotic character.
Whether in the British consular service on his Congo or Putumayo missions, or in the service of Irish nationalism, it is noteworthy that those he acted for rarely reined him in.
On the contrary, he was allowed a discretion given to few others. His strength of personality and his drive, which took him to countries where few others would have gone, wrung respect and even acclaim from his sponsors at the time.
Prof Reinhard Doerries in Prelude to the Easter Rising. Sir Roger Casement in Imperial Germany (Cass, 2000) published official correspondence between Casement and Count Georg von Wedel at the English desk in the German foreign office and his colleague Richard Meyer.
This has now been supplemented by a cache of private papers handed over in 1969 by a Co Clare solicitor, Ignatius Houlihan, whose daughter spoke eloquently at the opening of a new exhibition. He acted for Count Blücher, a long-time friend of Casement and a descendant of the Prussian marshal whose late arrival turned round the fortunes of the battle of Waterloo. In an early act of Franco-German reconciliation, Count Blücher bequeathed the emperor's carriage, earlier offered to Houlihan, to the prince Napoleon.
After the first war Blücher established a factory at Shannon. He died in Ennis in 1931 and is buried in Drumcliff cemetery.
The Clare county archivist Róisín Berry has with great enterprise organised a month-long exhibition on Casement in Germany in the spacious county museum in Ennis, one of a number of cultural assets in the town that benefited from Síle de Valera's tenure as arts minister.
A splendid souvenir catalogue has been prepared with many facsimiles and illustrations, and an article by Casement scholar Angus Mitchell, whose editions of the 1910 and 1911 "white" diaries are richly inspiring.
In his humanitarian work, Casement exposed de facto systems of slavery backed by casual genocide.
In 1910 he metaphorically saw "justice to unhappy, outraged people coming about through the Irishmen of the world". In 1912, through the influence of his work, an encyclical of Pius X called for more humane treatment of Indian peoples in South America.
Casement's work sharpened his awareness of imperialism, and he belonged to a key circle of northern nationalists, many Protestant, who were appalled at the diehard unionist and Tory opposition to Home Rule.
What comes out of these letters and pamphlets between 1913 and 1916 is something only half understood now - Britain and Ireland having changed so much in the meantime.
This was the determination by Britain, and before that England, to hold Ireland for what Peter Brooke in 1990 called "selfish strategic reasons".
Casement wanted to expose the moral cant when even that most liberal British chief secretary Augustine Birrell stated in 1907 that "separation was unthinkable, save in the event of some world cataclysm". The first World War was that cataclysm.
Casement's quotations from John Redmond are a reminder why Redmond left few followers.
For example, in a St Patrick's Day message in 1913, Redmond promised that a Home Rule Ireland would do "everything in our power to increase the strength and the glory of what will then be our Empire at long last, and, standing in support of the Empire, the strong arms and brave hearts of Irish soldiers and sailors".
Despite his unsuccessful attempts to recruit an Irish brigade in the German service from Irish prisoners of war, Casement did not glory in the first World War, and was critical of poor German propaganda and Germany's official indifference to its friends.
In his new year greetings in 1916 to Blücher, he wrote: "May it end with the world at peace."
The following month the plea was more urgent: "The only thing I really want is peace - peace all round. I am sick to death of all the rest - the hapless folly of the whole thing, the organised madness, the scientific insanity called 'war', 'victory', 'glory'; how vain and hopeless it is."
While committed to the cause of Ireland's independence, he had grave doubts about the Rising, though, like Tone in 1798, he felt he had to be there.
Appropriately, the principal GAA stadium in Belfast, Casement Park, is named after him. While the South was the beneficiary of the Irish revolution, Casement is a reminder of the importance of the Northern contribution.
His vision was of a broader cross-cultural nationalism, a goal that needs to be taken up again.