It is thought that he attended a local hedge school in Graiguecullen Queen's County, where he excelled in mathematics and accounting.

With the assistance of prominent local people, particularly John Alexander, a prominent Carlow miller, and Henry Parnell MP for Queen's County, Dargan began working with the Scottish engineer Thomas Telford on the Holyhead side of the London-Holyhead road.

[4] On 13 October 1828, William Dargan married Jane Arkinstall in the Anglican Church of St Michael & All Angels, Adbaston, Staffordshire.

[4] When Dargan came back to Ireland, he was occupied by minor construction projects, including rebuilding the main street of Banbridge and the 13 kilometers long Kilbeggan branch of the Grand Canal.

After a persistent effort of Dargan, Dublin & Kingstown Railway was able to be opened on 17 December 1834, with eight trains in each direction, at full capacity.

[7] Wishing to encourage the growth of flax, he then took a tract of land whose culture he devoted himself to, but owing to some mismanagement the enterprise entailed a heavy loss.

In 1860, continuing his branching out into different business ventures, Dargan brought the International Hotel in Bray from John Quin.

[9] As the committee of the 1853 Exhibition believed that this event would be self-financing, mainly relying on Dargan's contribution of £20,000, it was announced that there would be no cash donations taken.

[9] After the building costs had risen by the autumn of 1852, the committee was forced to make a public subscription, but Dargan offered another £6,000 as patronage.

[9] The British royal party arrived at Dun Laoghaire on 29 August 1853 at the purpose to attend the exhibition, and Queen Victoria personally met William Dargan.

All he has done has been done on the field of Industry & not of politics or Religion, without the Priest or factious conspiracy, without the promise of distant extraordinary advantages but with immediate apparent benefit.

A private undertaking, unaided by Govt, or any Commission with Royal Authority, made and erected at the sole expense of a single Individual, & this an Irish Road contractor, not long ago a common labourer himself, who had raised himself solely by his own industry & energy, - it deserves the greatest credit & is looked upon by the Irish with infinite self-satisfaction as an emblem of national hope".

At the close of the exhibition, the Irish National Gallery was built on Leinster Lawn, as a monument to Dargan, with a fine bronze statue of him in front of it, looking out upon Merrion Square.