St. Peter's is noted for its beautiful stained glass windows, particularly the west window and Harry Clarke's early masterpiece entitled 'The Adoration of the Sacred Heart'. The window depicts the Sacred Heart, Mary Magdalene and St. John the Evangelist. The window was installed in 1919.
St. Peters is richly decorated with Gothic embellishments, such as gargoyles, pinnacles, bosses and columns made from Newry granite. The principal entrance is in the front, which consists of double doors, deeply and richly recessed with Newry granite columns and moulded jambs, while the tympanum is elaborately carved, and has a statue of St. Peter in the centre, the whole surmounted with a crocketted gable and paneling.
In the early 19th century, Phibsborough was a crime-ridden suburb home to many families living in poverty. Proselytisers were roaming the streets discouraging Catholicism and converting people to Anglicanism. Eventually, the concern for the children of Phibsborough materialised and a Catholic school was built in 1826. Two of the priests who were then running the school, Rev. W. Young and Rev. W. Carroll, converted the top floor of the school into a chapel. In 1843, new schools were built to house the growing number of students. The second floor of the old school building was removed and the chapels length was augmented, leaving it 123 feet (37 m) long and 35 feet (11 m) high. It then became known as a church. Over time, more and more additions, augmentations and improvements were made to the church. In 1907, work on the spire commenced after Cardinal Moran of Australia commented on the lack of Catholic church spires in the Dublin skyline.