His time as a student was interrupted in 1946 due to a bout of tuberculosis of the kidney which forced him to spend 10 months in a health clinic in Switzerland.

Due to a relapse of his condition in 1947 that forced him to once again return to Switzerland, he missed his father's ascent to the office of Taoiseach in February 1948.

Beginning in 1956 until he died in 2011, Costello was the St Michael's House president, making him ultimately responsible for 170 centres across the Dublin region.

Considered the most radical of those who advised his father, Declan pushed against the austerity measures of Finance Minister Gerard Sweetman, who in time would become a great rival and adversary.

However, to their disappointment, Costello and his faction found the rest of Fine Gael slow and sluggish in response to their new ideas and thought that they were against the complex making of policies and the voicing of personal differences in public.

Following his father's retirement from politics in 1959, longstanding member James Dillon became leader of the party and Costello felt his ability to influence policy-making slipping away.

Regardless, in April 1964 Costello broke party protocols and circulated a proposal to radically alter Fine Gael's economic policies.

[4] Towards a Just Society called for a radical shift in the Irish economy to something closer to a social democratic mixed economy; it called for production objectives in the private sector, earnings and credit from banks to be controlled, no discrimination of women's wages, industrial school reformation, lower use of indirect taxes, free near-universal health care with the decision of what doctor to have and an educational system that permits moving to university despite being wealthy or not.

However, the Just Society's critique of public services in Ireland forced Fianna Fáil to alter its policies, and during the election, they made promises regarding housing, health and welfare.

[4][6] In 1968/1969, Costello successfully defended Sean Bourke against extradition to the United Kingdom about his assisting George Blake, the Soviet double agent, to escape from prison.

[6] In 1972 Costello unsuccessfully defended journalist Kevin O'Kelly against contempt of court charges, arising from a radio interview he conducted with Provisional IRA leader Seán Mac Stiofáin.

[4] Costello pushed, unsuccessfully, for family law in Ireland to be updated so that it would be more sympathetic to forsaken wives, mothers who were not married, and out-of-wedlock-born children, as well as other more liberal values.

In 1974, he also was the main person to promote a bill that would have liberalised the purchase of contraceptives in Ireland by allowing married couples to access them without barriers.

[9] As Attorney General, Costello refused requests by members of the coalition to have their constituents' minor criminal charges squashed, an established practice in Ireland.

Engaging the political right and tabloid press in the UK, Costello was able to prove in court that the British state was illegally using sensory deprivation techniques on prisoners.

[4] For having created the Office of the Director of Public Prosecutions and the Law Reform Commission, Costello has been called the "most consequential attorney general in the state's history".

[4] One tendency for which Costello was noted, was for early case decisions as to the rights and wrongs of each party's intentions, and encouraging the applicable barrister to make a satisfactory legal argument.

[4] Following the Whiddy Island disaster in 1979, in which 50 people lost their lives in an oil tanker explosion, Costello was appointed to lead a tribunal of inquiry to investigate the matter.

His report on the incident was highly critical of the two international companies responsible for handling the tanker (TotalEnergies and Gulf Oil), but also of a terminal controller on-site as well as the Irish authorities for failing to supervise safety practices.

[6] After the Widdy Island investigation, Costello was appointed chairman of two committees: one created for the development of a national youth policy, and the other to guide the charity sector's governance; both produced comprehensive reports but their findings were effectively ignored.

[4] In 1985 Costello upheld the firing of Eileen Flynn, an unmarried woman who had given birth to a child while working as a teacher at a Catholic-run school.

He ultimately supported the nuns who ran the school and their contention that Flynn's "conduct was capable of damaging" their efforts to uphold Catholic "norms of behaviour".

[6] Costello's decisions as a judge were informed by his belief "that the Irish constitution was best understood in the light of its Christian preamble and of passages acknowledging an ethical order superior to formal law."

In O'Reilly v. Limerick Corporation (1989), Costello ruled that the courts could not adjudicate over the state's distribution of public resources, as this required specialist knowledge.

[4] In February 1992, the Attorney General, Harry Whelehan, sought an injunction from the High Court to restrain a 14-year-old girl, known as "X", who was a victim of statutory rape, from travelling outside the State to obtain an abortion.

[6] The case came before Costello, who was forced to weigh the argument that there was a high probability of "X" committing suicide unless she were permitted to travel against the "right to life of the unborn" contained in the constitution following the Eighth Amendment in 1983.

[4] Costello's final act as a public figure was "to head an inquiry into the tax evasion conspiracy directed by the Guinness and Mahon Bank".

[4] Costello has been described as a "devotee" of the Catholic philosopher Jacques Maritain, a believer that "human rights existed to serve the common good rather than individual autonomy".

However, the match was never to come about as Bouvier continued her travels onwards to Scotland, and later returned to the United States while Costello married Joan Fitzsimons in 1953.