He was first elected to Dáil Éireann at the 1977 general election, and held his seat until the 1987 general election when he lost it to Brendan Howlin of the Labour Party. He was re-elected at the 1989 general election, at the expense of party colleague Avril Doyle, but she regained her seat at the 1992 general election. D'Arcy was then elected to the 20th Seanad on the Agricultural Panel.
At the 1997 general election he was returned to the 28th Dáil, again unseating Avril Doyle. He lost his seat again at the 2002 general election, this time to the independent candidate Liam Twomey, who later joined Fine Gael.
In 1981, in Garret FitzGerald's first government, D'Arcy was appointed a Minister of State for Agriculture in the 22nd Dáil. In the 1982–1987 FitzGerald government he was Minister of State for Fisheries until his removal in February 1986 in the government reshuffle.
His son Michael W. D'Arcy was elected to the Dáil at the 2007 general election, representing Wexford, and repeated the feat at the 2016 general election; from 2011 to 2016 he was a member of Seanad Éireann.