In 1919, Father Sullivan was appointed Rector of the Juniorate and Retreat House at Rathfarnham Castle on the outskirts of Dublin, an office which he held until his death in May 1924. His main duty was the care of about twenty young religious, who were engaged in their University studies.
There are many testimonies to Father Sullivan’s holiness at this period of his life. Frank Bradshaw, a domestic servant, recalled how he invariably found Father Sullivan kneeling before his crucifix whenever he entered the room.
For many years, Father Sullivan conducted a widespread apostolate of visiting the sick and suffering. On top of all his priestly duties, he found the time to regularly visit the needy, comfort them and administer the Sacraments to them. He visited Martin Connolly, formerly head cook in the College kitchen, once a week for about a year, and twice a year, when his condition worsened. He visited the former tailor to the College, William Byrne, for three years, during the third almost every day and towards the end twice a day.
By 1933, Father Sullivan’s health had started to deteriorate. He was now 72 years old. On February 4th, about a fortnight before his death, he developed a swelling in the elbow, which proved to be bursitis. On February 17th, he was suddenly attacked by a violent internal pain and was removed to hospital.
He spent the last two days of his life in continual prayer. One of the nuns recalls, “I think that what most struck me and those who were privileged to nurse Father John Sullivan during his last illness was his perfect conformity to God’s will and his marvellous patience.” On both days he received Viaticum. He died at 11 p.m on Sunday, 19th February 1933.