Back in the 1940s, Finglas was a small village about five miles from Dublin city. By the 1950s however, houses were being built on all sides of the Tolka. These houses were bringing a lot of children to the area. There were also buses full of children coming from East Wall and Gardiner Street to attend school. The school in Finglas became so overcrowded, they had to make pans to build a new school. In March 1956 Monsignor Glennon turned the first sod on the site of St. Fergal's on Cappagh Road. 

St. Fergal's opened it's doors on 1st September 1957. The official blessing took place that December. The mass was held in the school hall and presided over by Archbishop John Charles McQuaid. 

By now the ancient parish of Finglas had grown too large, so it was divided into two parts. St. Fergal's was firmly in the West of the parish, it's ties with St. Canice's in the East now severed. In 1959 the church of the Incarnation was built to serve the needs of the West. This church was known as tine 'Tin Church'. It was a temorary measure awaiting the arrival of a 'proper' church. 

In 1967, St. Fergal's school had 28 teachers and 1200 boys, many in prefabs in the field at the back of the school. The boys who were coming from the Barry area departed with Owen Sweeney and this was the start of St. Kevin's school. 

St. Fergal's school had an award winning choir, under it's renowned director Liam Hanley. They performed at many school and church occasions, including the official blessing of the school in 1957.