While Dubliners enjoyed the spring sunshine on Easter Sunday 1916 and the British military repaired to the Fairyhouse races, one man felt uneasy.
In his home in central Dublin, John Dillon, a shrewd Home Rule MP, committed his fears to paper: "Dublin is full of most extraordinary rumours and I have no doubt that the Clan men [ie, the Irish Republican Brotherhood] are planning some devilish business."
To most Irish people the Easter Rising came as a 'bolt from the blue'. Many saw it as 'a wicked German plot'. Yet its complex strands can be traced back over the previous 20 years of Irish history.
Perhaps the most important single factor in the making of '1916' was the steady growth of a new advanced form of nationalism in the years after Parnell's death in 1891.
In many ways a reaction to 'the death of Ireland's hopes', this 'New Nationalism' revealed itself in a rash of small dynamic groups ranging from literary societies through the Gaelic League and the GAA to Sinn Féin and the 'phoenix flame' of the IRB.
Together, they were to act as a ferment in the mind of a generation .
Of all these disparate strands, by far the most influential was the Gaelic League, founded in 1893 by the Protestant nationalist Douglas Hyde and Eoin MacNeill from the Antrim Glens.
The movement's dream was of a Gaelic-speaking independent Ireland and it soon became a 'school for separatists', creating the atmosphere for the Easter Rising.
Two other groups shaped by the Gaelic League's philosophy were Arthur Griffith's tiny but vital Sinn Féin movement and the Irish Republican Brotherhood.
Sinn Féin ('We Ourselves'), formed in 1905, argued that an independent Ireland could unite Orange and Green by retaining the Crown as a 'personal link' between Britain and Ireland.
Such a Hungarian-style 'dual monarchy' could only be achieved by a policy of passive resistance.
Official nationalism mocked Griffith's 'green Hungarian band' but it inspired the republican 'Dungannon Clubs', founded in Belfast by two IRB men, Denis McCullough, a Falls Road Catholic, and Bulmer Hobson, a Holywood Quaker.
Indeed, much of the background planning for 1916 took place in the gas-lit attics of Belfast after 1905.
It was there that Hobson and McCullough revitalised the IRB, then almost a dying force, and recommitted it to the United Irishman's slogan, 'England's difficulty is Ireland's opportunity'.
The Belfast 'young Turks' were joined by Sean MacDiarmada, a future 1916 signatory who worked as a Belfast tram-driver, and the incorruptible old Fenian, Tom Clarke, who hailed from Dungannon.
Yet these revolutionary stirrings seemed insignificant at a time when the Home Rule Party not only dominated Irish nationalism but seemed set to achieve all-Ireland Home Rule.
Even Pearse, then a cultural nationalist, supported the Home Rule Bill in 1912.
But the two years before the First World War were to see the emergence of determined Ulster Unionist resistance in the shape of the armed Ulster Volunteer Force.
Ironically, 'Carson's Army' was to 'relight the Fenian flame', presenting the IRB with a heaven-sent opportunity.
When MacNeill welcomed Carson's defiance of British authority in his article, 'The North Began' in 1913, the IRB urged him to form an Irish Volunteer Force on UVF lines.
As the IVF swelled to 180,000 members, drilled and armed, the countdown to the Easter Rising had begun.
By 1914 the Home Rulers' credibility had been badly damaged by the threat of partition. A turning point came in September when the IVF split over Redmond's support for recruitment in the First World War. In protest, a small but active section under MacNeill broke away and passed into the hands of the IRB which was to use it as the strike force for the now imminent insurrection.
Sir Roger Casement, a former British diplomat and nationalist convert, was despatched to Germany to seek military aid; a military council was established and, by January 1916, had been expanded to include the socialist James Connolly.
Crucially, the plot remained 'the conspiracy of a minority' within the IRB, centred on Clarke, MacDiarmada and their comrades.
The insurgent leaders could now rely on the support of the breakaway Volunteers (about 11,000 men) and Connolly's 200-strong Irish Citizen Army in the revolt planned for Easter.
The rising was originally intended as a successful national revolt. It was only when the best-laid plans began to go wrong over the Easter weekend with the capture of Casement and MacNeill's frenzied attempt to prevent a rising which he felt was suicidal that the element of 'blood sacrifice' became paramount.
The revolutionary council fell back on the 'Dublin Plan' for a symbolic stand against British rule and, on Easter Monday, around 1,000 insurgents seized defensive positions.
With the struggle effectively narrowed to the capital, Pearse and his comrades knew they had no hope of military success.
But they shrewdly calculated that their armed uprising would provoke the British into 'harsh reprisals' and give their cause its elixir of life.
The insurgents judged accurately. The rising at first engendered strong feelings of hostility among Irish nationalists, many of whose relatives were fighting on the Western Front.
But its aftermath and, in particular, the execution of 16 of the leaders, transformed public opinion almost over-night.
As one observer wrote: "A few unknown men, shot in a barrack yard, had embittered a whole nation."
The executions, accompanied by martial law, internment and mass deportation, were to convert nationalist Ireland to the idea of a totally independent state, partly achieved after the War of Independence of 1919-21.
Yet the men of Easter Week had claimed the sacred right of an armed minority to use violence in the name of the Irish people.
This was a fateful precedent and has ensured the continued presence of the 1916 ghosts 'MacDonagh's bony thumb' in Yeats's phrase on the Irish political stage down to our own times.