In early 1939 Gallagher pooled his savings with his brothers Joe, Dan, Hubert, and James, to put a down payment on two trucks.
They used these to transport materials that were used to build factories, runways and raid shelters for the British World War II preparations.
At first they worked out of Liverpool, but when the air raids on British cities began in 1940, they moved to a shale pit in Lancashire which they leased.
In Ireland they formed a partnership and traded as Messrs Gallagher, Roarty and Furlong, and began work on small local-authority housing contracts in Dublin.
They primarily worked in Dublin, and while still taking on some local-authority contracts, the brothers preferred to build on land they owned to maximise their profits.
[1] In response to supply issues, the Gallaghers diversified into other fields including joinery with Gowna Wood Industries, at Tubbercurry founded around 1950, and in 1952 establishing the Dublin plumbing and heating firm, P. J. Matthews Ltd.
Gallagher retired to his farm and mansion, "Hollywood Rath", Mulhuddart, County Dublin where he bred cattle.
In 1965, Gallagher ran unsuccessfully in the senate elections as a Fianna Fáil candidate, and instead became a significant party donor.
Along with fellow property developers P. V. Doyle and John Byrne,[1] Gallagher employed Desmond Fitzgerald as an architect, Des Traynor as his accountant, and Christopher Gore-Grimes as his solicitor.
Customers of Merchant Banking Limited were mostly Irish emigrants in Great Britain and the United States who deposited money in Ireland to avoid tax in their resident countries.
Fianna Fáil oversaw a new policy which aimed to ease the housing crisis by incentivising private property development and promoting home ownership.
[1] He continued to supply the Dublin housing market with affordable suburban homes, but also more upmarket residential developments in Castleknock.
He also exploited Irish urban land laws which allowed him to retain residual property claims, meaning he could levy ground rents on those who purchased his houses.
With the houses declining, the Corporation ordered their demolition on the grounds of safety, and large areas of the south side of the square were levelled to make way for an office development.
In particular Desmond and Mariga Guinness stymied his development by buying a house in the middle of the square's south side in 1964, number 50.
He agreed to be interviewed for the programme, and later bowed to political pressure to sell his Mountjoy Square properties to the Irish Georgian Society, at a profit.
[8] During the 1969 general election, this transaction became a major issue, and Gallagher was mentioned numerous times in Dáil debates.
In an effort to redeem his reputation, and that of his political allies, Gallagher donated £10,000 to the Meath Hospital for the construction of a new administration building.
[9] As an employer to 750 people directly, and a further 900 contract workers, Gallagher remained an important ally to the Fianna Fáil leadership.
He expanded his reach, starting commercial developments such as Donaghmede shopping centre, and building housing estates in Cork, Galway, Limerick, Sligo, and Waterford, finishing 1,000 homes in 1973 alone.