The Conscription Crisis of 1918 stemmed from a move by the Government of the United Kingdom to impose conscription in Ireland, and contributed to pivotal events in early 20th century politics in Ireland, galvanising popular support for parties favouring separation from the United Kingdom.
From early 1918 the British Army were dangerously short of troops for the Western Front. In the German Spring Offensive of 1918, German troops broke through the Allied lines in several sectors of the front in France, with a local advantage in numbers of four to one, putting severe strain on the Allied armies. The British Army, in one day, suffered a stunning setback, with the enemy overrunning ninety-eight square miles of territory, and penetrating, at the furthest point, to a depth of four and a half miles.
In addressing this grave military situation, the British Government decided to extend conscription to Ireland, (conscription in Britain started in January 1916), as an untapped reservoir of manpower for the front through a new Military Service Bill, as well as proposing a new Home Rule Bill. This had the effect of alienating both Nationalist and Unionists in Ireland. Despite opposition from the entire Irish Parliamentary Party, conscription for Ireland was voted through at Westminster.
Though large numbers of Irish men had willingly joined Irish regiments and Irish divisions of the New British Army at the outbreak of war in 1914, the likelihood of enforced conscription created a backlash. The reaction was based particularly on the fact that implementation of the Government of Ireland Act 1914 (as previously recommended in March by the Irish Convention) was controversially linked with a "dual policy" enactment of the Military Service Bill. The linking of conscription and Home Rule outraged the Irish parties at Westminster, (including the IPP, AFIL and others) who walked out in protest and returned to Ireland to organise opposition.
On 18 April 1918, acting on a resolution of Dublin Corporation, the Lord Mayor of Dublin (Lawrence O'Neill) held a conference at the Mansion House, Dublin. The Irish Anti-Conscription Committee was convened to devise plans to resist conscription, and represented different sections of nationalist opinion: John Dillon and Joseph Devlin for the Irish Parliamentary Party, Eamon de Valera and Arthur Griffith for Sinn Féin, William O'Brien and Timothy Michael Healy for the All-for-Ireland Party and Michael Egan, Thomas Johnson and W X O'Brien representing Labour and the Trade Unions.
On the evening of the same day, the Catholic bishops were holding their annual meeting at Maynooth (with a similar agenda, to deliver a "Statement on Conscription") and they met a delegation from the Mansion House Conference.
"Denying the right of the British government to enforce compulsory service in this country, we pledge ourselves solemnly to one another to resist conscription by the most effective means at our disposal."
Following their representation at the Mansion House, the labour movement made its own immediate and distinctive contribution to the anti-conscription campaign. A general strike was called in protest, and on 23 April 1918, work was stopped in railways, docks, factories, mills, theatres, cinemas, trams, public services, shipyards, newspapers, shops, and even Government munitions factories. The strike was described as "complete and entire, an unprecedented event outside the continental countries".
In the following weeks, anti-conscription rallies were held nationwide, with 15,000 people attending a meeting in County Roscommon at the start of May, where John Dillon, leader of the Irish Parliamentary Party and Eamon de Valera of Sinn Féin shared the platform in a united cause. This in itself is notable as, while sharing nationalist views, Dillon and de Valera's parties had previously been divided in opinion as to the best means for devolution from the UK, and would subsequently be divided by the Anglo-Irish Treaty. However, all were galvanised to collective action on the conscription issue.
In the fourth year of a war ostensibly begun for the defence of small nations, a law conscribing the manhood of Ireland has been passed, in defiance of the wishes of our people. .... To warrant the coercive statue, no recourse was had to the electorate of Britain, much less to that in Ireland. Yet the measure was forced through within a week, despite the votes of Irish representatives and under a system of closure never applied to the debates, which established conscription for Great Britain on a milder basis.
Nervous of growing unrest, and still with dire need to progress conscription in Ireland, Lloyd George's government undertook several initiatives to quell the backlash.
As Sinn Féin were publicly perceived to be key instigators of anti-government and anti-conscription feeling, the viceroy Lord French, claiming evidence of a treasonable plot between Sinn Féin and the Germans, ordered the arrest of seventy-three Sinn Féin leaders, including Griffith and de Valera, on 17 May. This heavy-handed response by the Dublin Castle authorities did little to defuse the situation however. (In fact, a lack of evidence meant the German Plot was little believed in the UK, Ireland or the US, and aggravated opinion and Sinn Féin support.)
Simultaneously a more subtle effort (and possibly with more potential for success) was undertaken from the offices of Lord Northcliffe under the Minister of Information. The "Hay Plan" was conceived by Stuart Hay—a British Army Captain—who was under orders to establish a proposal to work around widespread anti-conscription feeling and persuade Irish Nationalists to join the French army (initially as labourers in specialised battalions). Hay's plan relied on the power of the Catholic Church in Ireland (and empathy among Irish Catholics for Catholic Belgium and France which were under German occupation), to sway opinion:
If the church were definitely or even in a large measure converted and the support it has given to disloyal elements be not taken away but thrown on to the other side in the controversy [the conscription crisis], much would be done for the future of the peace in Ireland.
The plan simply called for a letter (drafted by Hay, and approved by Edward Shortt, then Chief Secretary for Ireland) to be sent by the French Primate to the Irish bishops, requesting that they soften their opposition to conscription to aid the war effort in France.
Despite some progress in August in persuading Primate of All Ireland Cardinal Logue through these means, the "Hay Plan" was delayed (and ultimately stymied) by complications in diplomatic channels and by political rivalries.
As a result, recruitment efforts through September and October continued to have very limited success, and by the Armistice in November (marking the end of World War I) conscription remaining unimplemented in Ireland.
By June 1918 it had become apparent to most observers in Britain and Ireland that due to America's engagement the tide of war had changed in favour of the Allied armies in Europe, and by 20 June the Government had dropped its conscription and home rule plans. However the legacy of the crisis remained. 
Completely ineffectual as a means to bolster battalions in France, the events surrounding the Conscription Crisis were disastrous for the Dublin Castle authorities, and for the more moderate nationalist parties in Ireland. The delay in finding a resolution to the home rule issue, partly caused by the war, and exaggerated by the Conscription Crisis in Ireland, increased support for Sinn Féin.
Sinn Féin's association (in the public perception at least) with the 1916 Easter Rising and the anti-conscription movement, directly and indirectly led on to their landslide victory over (and effective elimination of) the Irish Parliamentary Party, the formation of the first Dáil Éireann and in turn to the outbreak of the Anglo-Irish War in 1919. (See: Aftermath of World War I - United Kingdom and Irish (UK) general election, 1918).
This opposition also led in part to Sinn Féin being ignored by the subsequent victors at the Paris Peace Conference in 1919, despite its electoral success. It appointed representatives who moved to Paris and several times requested a place at the conference, with recognition of the Irish Republic, but never received a reply.
It should be noted that the bulk of opposition to the Great War in Ireland was to compulsory conscription, not to the war nor to voluntary enlistment in the British Army. In fact, many Irish, including Irish Americans supported the war and Irish involvement.  See
Support and enlistment was more prominent amongst Irish unionist and Protestant traditions, however, nationalist and Catholic enlistment was also common as the war was seen to be fought in defence of smaller Catholic countries (like occupied Belgium). In this cause, those who would later become detractors of conscription (including John Dillon, William O'Brien and the Catholic bishops) were prominent on recruitment platforms at the outbreak of the war.
In all, 200,000 to 300,000 Irishmen served with British forces during the Great War, and, of the 680,000 fatalities from the United Kingdom of Great Britain and Ireland, some 40,000 to 49,000 were from Ireland.
The conscription crisis was a watershed in Ulster Unionism's relations with Nationalist Ireland. As Unionist leader James Craig explained, Ulster Unionists loved Ireland just as much as any Irish Party or Sinn Féin supporter, and if they accepted conscription in Ireland allowing Ireland take a full share of 'national responsibility' at a time of crisis, Nationalist would do more to secure the affection of the people of Great Britain and the Empire, and to win Ulster to join with them in a home rule parliament on a future occasion, than by any other step they could have taken. Ireland would then be regarded as part and parcel of the British Empire, taking her full share of its responsibilities. As this did not happen, for Ulster Unionists the conscription crisis was the final confirmation that the aspirations of Nationalists and Unionists were unrecompatible.(sic)
"...it seems to me a strangely wanton thing that England, for the sake of 50,000 Irish soldiers, is prepared to hollow another trench between the countries and fill it with blood."
"...women and children will stand in front of their men and receive the bullets, rather than let them be taken to the front."
...the Chief Secretary for Ireland felt that . . . "Irish conscripts would be as useful as conscripted Germans."