Clancy spent three years as a Classics teacher at Holy Cross School, Tralee, where he married Margaret Louise Hickie (d. 1912) of Newcastle West, County Limerick, in 1868.
In this role he wrote or edited dozens of pamphlets, many of them attacking the regime of coercion introduced by Arthur Balfour as Chief Secretary for Ireland after the Conservative Party returned to power in 1886.
Thereafter he worked closely with John Redmond, who led the small Parnellite group and after the 1900 general election the re-united Irish Parliamentary Party.
He acted as spokesman for the Irish Party in support of the Trade Disputes Bills of 1904 and 1906 that restored the effective right to strike—which had been undermined by the Taff Vale Case of 1901.
After the Liberals' return to office in the 1906 general election, Clancy helped get inscribed on the Statute Book Acts of considerable importance.
But the 1909 budget was also unpopular in Ireland, because of changes to alcohol taxes and death duties, the latter affecting the very farmers whom the Irish Party itself had successfully campaigned to make owners of their farms.
A delicate balance needed to be trodden and it fell to Clancy, by now the Irish Party's finance spokesman, to deal with the problem.
Following the Easter Rising of 1916, misjudgments by the British government bolstered support for Sinn Féin, the broad movement campaigning for an independent Republic, and events slipped out of the Irish Parliamentary leaders' control.
Although the Convention produced a majority report, the consensus did not include the Northern Protestants and Lloyd George subsequently went ahead with legislation for partition under the Fourth Home Rule Act.
In that election he was defeated by more than two to one by the Sinn Féin candidate, Frank Lawless, the Parliamentary Party swept aside and only winning a disproportionate six contested seats, on 21.7% of the national vote in Ireland.