The Cathedral of Saints Peter and Paul, built between 1828 and 1843 and raised to cathedral status in 1990, sits at the south end of O'Connell Street. Its tall granite spire — added later, in 1874 — is the most visible landmark in Ennis from any approach by road.
The interior is plain Gothic Revival, with limestone columns and an exposed timber roof. The stained-glass east window depicts the Crucifixion in deep reds and blues. The cathedral is the seat of the Catholic Diocese of Killaloe; daily masses keep the building in use, but it is generally open between services for visitors.