[4] At St Teresa's, the nine-year-old Drennan was introduced by a teacher to the book Promises to Keep by Dr Tom Dooley about the Vietnam War.

[5] He studied for the priesthood at Holy Cross Seminary, Mosgiel, for two years[6] before being sent by his bishop, Basil Meeking of Christchurch, to Rome to complete his training at the Pontifical Urbaniana University.

After Drennan's post-graduate study in Rome, he was on the staff of Holy Cross Seminary and Good Shepherd College in Auckland.

Just before leaving the Vatican, he accompanied the Pope on a visit to the Czech Republic, during which they discussed the Church in New Zealand.

[8] Drennan was appointed coadjutor bishop of Palmerston North by Pope Benedict XVI on 22 February 2011.

[9] He chose as his motto caritas congaudet veritati (love delights in the truth), taken from the First Letter of St Paul to the Corinthians, chapter 13.

[12] On 7 March 2015, Drennan joined with other protestors in a demonstration in Palmerston North in relation to the Trans-Pacific Partnership (TPP).