[2]: 34 Born at 10 Hamill Street in the Lower Falls area of Belfast, he was the fifth child of Charles Devlin (c.1839-1906), who was a self-employed 'jarvey',[2]: 24  and his wife Elizabeth King (c.1841-1902), who sold groceries from their home; both were Catholics.

[3] While working briefly as a clerk and in a pub, he showed an early gift for public speaking[3] when he became chairman of a debating society founded in 1886 to commemorate the first Irish nationalist election victory in West Belfast.

Devlin then worked at Samuel Young MP's brewery company, for which he was assistant manager of Kelly's Cellars, a Belfast pub, until 1902.

Devlin had risen in the ranks of the League from being a local Nationalist organiser in Belfast to becoming the only newcomer to the parliamentary party who was accepted politically, as an equal by the established leaders.

[7] He became a distinguished parliamentarian and had reached the top by the skilful use of two remarkable talents, his persuasive and very powerful oratory, and secondly, that he was a great organisation man, not merely as General Secretary of the United Irish League, but because he also dominated the Ancient Order of Hibernians.

[2]: 99  Devlin represented the main urban and national business interests, which contrasted with his advocacy of social reforms when he took up labour issues especially working conditions in the linen mills and textile trades.

Devlin became governor of the nationalist hinterland after his AOH political machinery rapidly saturated the country, acting through the UIL as the militant support organisation of the Irish Party.

Redmond's plan was that, post-war, an intended 'Irish Brigade' and the National Volunteers would provide the basis for an Irish army, capable of enforcing Home Rule on reluctant Ulster Unionists.

[2]: 153  The Republican James Connolly wrote of the 'Wee Bottlewasher' as 'a recruiting sergeant luring to their deaths the men who trusted him and voted him into power.

Although, when he tried to bring up the Croke Park killings that occurred on Bloody Sunday at Westminster, he was shouted down and physically attacked by Conservative MP John Elsdale Molson; the Speaker had to suspend the sitting.

This is not a scattered minority...it is the story of weeping women, hungry children, hunted men, homeless in England, houseless in Ireland.

What will we get when they are armed with Britain's rifles, when they are clothed with the authority of government, when they have cast round them the Imperial garb, what mercy, what pity, much less justice or liberty, will be conceded to us then?

Devlin was re-elected by a large majority in Belfast West in 1925 when he decided to lead his small party out of abstentionism and sat for the first time in the Parliament of Northern Ireland as head of a powerless opposition.

In 1927, Devlin was urged by Alfie Byrne to stand in the Irish general election (for a seat in the Dublin parliament) but he refused saying "If I do that, the poor people of Belfast, who have stood by me loyally for the past thirty years and who are undergoing the tortures of the damned will, I fear, think I am taking this opportunity of slipping out of a difficult position.

In later years he was comfortably off as director of the Distillery Company and chairman of the Irish News, and enjoyed organising summer fêtes – "days of delight" – for Belfast children.