There is an assumption that most Irish Nationalists were disposed to Home Rule because all Nationalist Mps elected to Westminster were either members of the IPP, the All For Ireland League or alternatively Independent Nationalists. In contrast, Sinn Fein and their manifesto for an independent Ireland attracted little or no support from the voting public. However, this analysis ignores the fact that Nationalist MPs were elected from a narrow subset of the population because universal suffrage did not exist. Voting was restricted to upper class males, mainly wealthy business men and property owners. Most were reluctant to change the status quo and wished to maintain some sort of link with Britain and the Empire.
This is borne out by the voting figures in the 1910 and 1918 elections. The total vote for Nationalist MPs was 114,485 in the January 1910 election and 121,379 in the December 1910 election. In 1918 the vote for the IPP and other Nationalist Mps almost doubled to 234,681, as a result of the extension of the suffrage. Most of these votes would have come from upper class women. The IPP in fact held on to its existing voter base. The IPP lost because it failed to attract new voters from the newly enfranchised lower classes who were republican in outlook and favoured independence.
The 1918 election offered this hitherto invisible Republican constituency their first chance to vote for independence courtesy of Sinn Fein and they took it. This is the primary reason for the Sinn Fein victory.
It is fair to assume that most nationalists would have voted for independence in 1880 if universal suffrage had existed, most Nationalists would have voted for independence in 1900 if universal suffrage had existed and most Nationalists would have voted for independence in 1910 if universal suffrage had existed.
In fact, it was a forgone conclusion that Sinn Fein or any other party that fought the 1918 election on an independence platform would get most of the Nationalist vote and this would have happened regardless of the aftermath of the 1916 rising, the conscription crises, anti-war sentiment or any other reason put forward by historians trying to explain the seismic shift in attitudes. There was no seismic shift. Republicans simply got to vote for the first time.