At Russell's request, a leading volunteer within the organisation, Seamus O'Donovan, authored a strategic plan to extend the IRA's military campaign to mainland Britain with the aim to pressure the British government to withdraw from Northern Ireland.

O'Donovan's strategy was to continually target economic, military, and civic infrastructure within mainland Britain, with the targets to be located across the entire mainland and the methods not simply limited to bombings, but to include other forms of sabotage[7] and those committing the acts to be mainland-based Irish citizens residing "[within] and around centres of population" where they could operate with little chance of their acts drawing attention.

Initially active in Southern England, McCormick relocated to Coventry in either late May or early June 1939,[12] where he lodged with a young Irish-born couple: Joseph Hewitt, his wife Mary, their baby daughter Brigid Mary, and Hewitt's mother-in-law, Brigid O'Hara, at 25 Clara Street on the outskirts of Stoke Heath, paying 23s per week for bed and breakfast.

[3][b] Construction of the bomb itself was completed in the early hours of the following morning, with the device attached to a timer and concealed within a cardboard box wrapped in brown paper and tied with string.

[4] At approximately 13:40 on 25 August 1939, an unknown IRA member[c] left the bicycle containing the bomb standing against a kerb outside Astley's shop in the busy Broadgate area of Coventry.

[3][4] Contemporary news reports describe the vicinity of the explosion as resembling a "miniature battlefield", and indicate uninjured bystanders covered the bodies of the deceased with overcoats.

[20] The five who died in the Coventry bombing were Elsie Ansell, 21; John Corbett Arnott, 15; Rex Gentle, 30; James Clay, 82; and Gwilym Rowlands, 50.

[21][22] Ansell—the closest individual to the bicycle—was browsing a display of jewellery in a H. Samuel shop window at the time of the explosion; her facial and skull injuries were so extensive her body was only identifiable by her engagement ring.

Arnott and Gentle were both employees at a nearby WH Smiths; both were returning to their workplace from a lunch break at the time of the explosion and were also killed instantly.

Rowlands was a council employee working with a colleague close to the scene of the explosion; his cause of death was ruled as haemorrhaging due to multiple injuries.

"[4] The chief constable of Coventry was also forced to issue a public denial of rumors he was Irish, stating that he was a "perfectly good Somerset man" who had never even set foot in Ireland.

His arrest resulted from an independent Special Branch police raid upon a flat in Leinster Gardens, West London, in relation to numerous local thefts of bicycles by the tenants of the property which investigators suspected of being used in the transportation of explosives around the city by the IRA.

[24] A search of Barnes's address uncovered ample incriminating evidence with regards to the Coventry bombing, including a suitcase bearing traces of potassium chlorate, and an unposted letter to an acquaintance in Ireland in which Barnes detailed his involvement in and expectation of a forthcoming "spectacular" bombing and concluding with "I am coming back from Coventry tonight 11:30, so by the time you get this, the [newspapers] should have some news.

This search also uncovered a suitcase bearing traces of potassium chlorate, plus tools and equipment sufficient for the purpose of constructing bombs and incendiary devices such as pincers, soldering material, and insulating tape.

The following day, both Brigid O'Hara and Mary Hewitt provided written statements in which they admitted several "strange men" had visited McCormick at their house in recent weeks, often bringing suitcases and substances they had suspected were explosives.

In addition, shortly after midday on 25 August, she had observed McCormick and a man she did not know the name of working with tools on a wooden box in her own bedroom.

[28] When shown a photograph of Barnes, both women subsequently identified him as having visited their home for the first time on 21 August—apparently to acquaint himself with McCormick and discuss the transportation of the explosives.

[31] Although the prosecution acknowledged none of the defendants had actually constructed or planted the bomb, the Crown contended all had played an active part in a conspiracy to endanger life; as such, all were charged with murder.

The two women admitted to holding suspicions of the "white powder" in McCormick's possession being explosives, whereas Joseph Hewitt claimed to have "no idea" of the substance's source.

[34] He also confirmed claims earlier made by his wife and mother-in-law upon the stand that both had asked him "more than once" in the weeks prior to the bombing to evict McCormick from the property, before testifying he had failed to do so.

They deliberated for just thirty-one minutes before reaching their verdicts:[37] Joseph and Mary Hewitt and Bridget O'Hara were acquitted of all charges; all three were subsequently deported to the Irish Republic under government security measures.

[38] Barnes and McCormick, however, were sentenced to death for their part in the bombing—[39][40] the jury agreeing with the prosecution's contention that the two had collectively constructed and assembled the bomb even if neither had intended for the device to detonate at the location it ultimately had.

"[42] Judge Singleton then donned his black cap and proclaimed: "Peter Barnes and James McCormick, the jury have returned a true verdict according to the evidence.

"[11] Prior to the executions, the Irish Taoiseach, Éamon de Valera, had repeatedly motioned for the sentences of Barnes and McCormick to be commuted; his final request was dismissed by the Secretary of State for Dominion Affairs, Anthony Eden, on 29 January 1940.

[47] The Coventry bomb was initially believed to have been originally destined to detonate at an electricity generating plant located on the outskirts of the city.

O'Sullivan also alleged that, en route, the bicycle used in the transportation had repeatedly gotten stuck in tram tracks, thus leading him to abandon the bike in Broadgate.

[3][54] O'Sullivan also revealed the reason he was not caught or charged with the bombing was because he had foreseen that authorities would arrest and interrogate all Irishmen attempting to board a ferry bound for Ireland via the portal town of Holyhead with a ticket purchased in Coventry or Birmingham in the hours or days following the explosion.

[5] On the centenary year of the Easter Rising, the Spirit of Irish Freedom Society Westmeath held a graveside commemoration to honour fallen republicans from the Midland Region.

At this service, the Thomas Allen Society led a cortège of over fifty people to the graveside of Barnes and McCormick in Ballyglass Cemetery, where wreaths were placed on the graves of both men.