It is thought that through his Fenian connections, he first met William O'Brien when he was conducting his investigative journalism in the Galtee Mountains on social conditions there in 1876–1877.

By 1886, Mandeville was the ex-officio chairman of the Mitchelstown board of guardians, and a close confidant of O'Brien who drew him into the Irish National Land League and reform politics.

The local economy of the time was suffering from continuous fall in butter prices, and intense resistance at the evictions taking place around Mitchelstown on the Kingston estate in 1881 to 1882 led to unrest and resentment among tenant farmers.

Mandeville chaired the local National League branch, and was eager to launch the Plan of Campaign, announced in October 1886, if appeals by tenants for 20% reduction in rent went unheeded.

[1][2] With threats that the dowager countess of Kingston was planning to rush through evictions ahead of the land legislation becoming law, Mandeville and O'Brien called on those attending a meeting in Mitchelstown on 9 and 11 August to prepare to defend their houses when outstanding warrants were executed.

This led to both being summoned to Mitchelstown petty sessions on 9 September under the Criminal Law and Procedure (Ireland) Act of 19 July 1887 which outlawed incitement against payment of rent.

O'Brien was transferred to the prison hospital swiftly when his health declined, Mandeville who was not as well known, was targeted to break his insistence on his political status.

He was repeatedly placed in solitary confinement, on a diet of coarse bread and cold water in draughty, unsanitary conditions, where he developed continual diarrhoea, painful rheumatism and a chronic sore throat.

[1][2] When members of the visiting committee leaked information about Mandeville's treatment to the press, a scandal broke out in late November 1887 across Ireland and England.

Balfour dismissed the decision as partial, but public outcry led to the establishment of a select committee on prison dress and other elements of the penal code.