Cullen was born at Prospect,
Narraghmore,
Athy,
County Kildare, one of 16 children of Hugh and Judith (Maher) Cullen, six of whom were from Hugh's first marriage. His first school was the
Quaker Shackleton School in nearby
Ballitore. Following the relaxation of some of the
Penal Laws, his father purchased some , giving him the status of a Catholic "strong farmer", a class that greatly influenced 19th-century Irish society. They were fervent in their Catholicism and fearful of the sort of social unrest that had led to the failed
1798 Rising. His great-nieces,
Mary and Elizabeth Cullen became nuns, and two great-nephews entered the priesthood. Cullen entered
St Patrick's College, Carlow, in 1816, and, in 1820, he proceeded to the
Pontifical Urban College in Rome, where his name is registered on the roll of students of 29 November 1820. 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. Vincenzo Pecci, the future
Pope Leo XIII, was present at the disputation. Cullen graduated a doctor of divinity. He was ordained in 1829. During his studies, Cullen acquired knowledge of classical and Oriental languages. He was later appointed to the chairs of Hebrew and Sacred Scripture in the schools of the Propaganda, and receiving at the same time the charge of the famed printing establishment of the
Congregation of Propaganda Fidei. 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. ==Rector of Pontifical Irish College==