A trained biblical theologian and scholar of ancient languages, Cullen crafted the formula for papal infallibility at the First Vatican Council.

Following the relaxation of some of the Penal Laws, his father purchased some 700 acres (2.8 km2), giving him the status of a Catholic "strong farmer", a class that greatly influenced 19th-century Irish society.

At the close of a distinguished course of studies, he was selected to hold a public disputation in the halls of the Propaganda on 11 September 1828, in 224 theses from all theology and ecclesiastical history.

The theological tournament was privileged in many ways, for Pope Leo XII, attended by his court, presided on the occasion, while no fewer than ten cardinals assisted at it, together with all the élite of ecclesiastical Rome.

This later charge he resigned in 1832, after being appointed Rector of the Pontifical Irish College in Rome, but during the short term of his administration, he published a standard edition of the Greek and Latin Lexicon of Benjamin Hedericus, which still holds its place in the Italian colleges; he also edited the Acts of the congregation in seven quarto volumes, as well as other important works.

[8] Cullen was appointed Roman Catholic Archbishop of Armagh on 19 December 1849 and consecrated by the Cardinal Prefect of Propaganda at the Irish College in Rome on 24 February 1850.

It occurred during the period of the debilitating Irish Famine which reduced the population of the country by over 2 million people through starvation, disease and emigration.

[10] Cullen was sent to Ireland to bring the Irish church into conformity with Roman canon law and usage[2] and to that end he recruited new clergy and orders of religious brothers and sisters.

[8] John Henry Newman, whom he had invited to be Rector of the Catholic University, complained that the Archbishop treated him and the laity not as equals but as his subjects.

He took part in the definition of the dogma of the Immaculate Conception of the Blessed Virgin Mary in 1854 and with the 18th centenary of the martyrdom of Saints Peter and Paul in 1867, when he stayed at the Irish College.

It was a matter of great delicacy, as promoters of the definition were split in various factions, some anxious to assign a wide range to the pope's decisions, and others would set forth in a somewhat indefinite way the papal prerogative.

[12] In political matters, Cullen made it a rule to support every measure, whatever its provenance, conducive to the interests of his vision for the Roman Catholic Church in Ireland.

He had fought in the American Civil War on the side of the Confederacy, and the British government was determined to deter other skilled military leaders from enlisting with revolutionaries.

Addressing a small gathering in the Crypt of the Pro-Cathedral at the reinterment, Archbishop Dermot Farrell prayed: “We come together to bring the body of Cardinal Paul Cullen, my predecessor, to its new resting place.

With his experience and friendships in Rome he was able to influence the choice of appointments to episcopal sees in Australia, New Zealand, South Africa, and Canada.

His relatives, friends, and students, referred to as "Cullenites", exerted great influence overseas, with his nephew, Patrick Francis Moran, archbishop of Sydney, one notable example.

[15] Father Thomas N. Burke, O.P., in a sermon at a solemn Requiem mass, the "Month's Mind" of 27 November 1878, said: "The guiding spirit animating, encouraging and directing the wonderful work of the Irish Catholic Church for the last twenty eight years was Paul, Cardinal Cullen.

[17] Although a devout Catholic herself, Mary Jane, wife of Jeremiah O'Donovan Rossa, wrote a blistering response in her poem, "Tis for the Glory of the Faith", to what she saw as Cullen's suggestion that Irish emigrants had the role of spreading Catholicism abroad.