The Rector at the time, Fr. Paul Mary Pakenham  decided to take the top story from the house an build a Church that would extend from the right side of the house and in line with it. The foundation stone was laid on 19th September and the Church was officially opened on the 18th December of the same year. The Church could accommodate 600 people.

Other Fathers were assigned to the new Retreat and plans were drawn up for the building of a larger house for the religious. It would be sited behind the present house but running to the left. The foundation stone for the new monastery was laid by Dr. Whelan, Bishop of Bombay on 13th June 1859. At this time Fr. Charles Houben C.P. has just arrived in the house. The building was a massive undertaking for its day as the picture indicates. The architect Mr. J.J. McCarthy drew up the plans and the builder was Mr. Meade and the estimated cost £12,000. The three storied building had an extension on the left side completing a 'L' shaped block. At the end of this extension was a transept. The bottom floor was the kitchen and dining room, the middle floor the bathrooms and sick room and the top floor was the Choir or private chapel of the religious. Eventually on the right of the front it was hoped to build a Church that would also run to the rear.

Misunderstanding arose with the builder who withdrew and the monks themselves took on the task of completing the building. Bro. Alphonsus Zeegers C.P. became Clerk of Works and Bro. John Walsh C.P became Master of Works. Employing local labour and craftsmen the building took four years and was opened on 8th September 1963. Today one marvels at their enterprise, courage and tremendous faith in attempting such a task. The finished product was proclaimed "the noblest religious house erected in these countries since the so-called 'reformation'." The old house was demolished.

The first Church of Blessed Paul of the Cross was now dwarfed by the new monastery. To accommodate the increasing numbers coming to the Church and to have a Church in harmony with the new monastery the plans for a new Church from Mr. J.J. McCarthy were accepted. The foundation stone was laid 29th June 1866 under the Rectorship of Fr. Alphonsus O'Neill C.P. Grave financial difficulties put a halt to the building. Fr. Pius Devine was Rector from 1869 to 1972 and then went to America to collect funds. Fr. Alphonsus O'Neill became Rector again in 1872 and re-started the job in 1873.  Building began to a modified design and five years later the Church of St. Paul of the Cross was dedicated on 28th April 1878. The Church facade was in line with the front of the monastery and the body of the Church stretched to the rear. 

Services were now transferred to the new Church but the old Church was still used particularly by Father Charles. It took on more the functions of a hall with plays and concerts being preformed there. Fr. Charles died in 1893. At the end of that year the old Church was demolished and the new cemetery designed. It was to be named the 'Father Charles Cemetery'. Those who had been buried beside the old Church were moved the short distance to the new cemetery. All the building left Mount Argus, nearly permanently in debt. In 1913 Fr. Sebastian Slean C.P., the Rector, ran a monster 'Ovada Bazaar' in Ballsbridge. The profits were sufficient to remove the debt.

In 1924 a major refurbishing of the Church took place under the direction of Bro. Michael C.P. It involved not just a repainting of the interior but also the addition of many decorative paintings with representations of Christ and the Apostles, the Coronation of Our Lady, and St. Paul of the Cross. When the 1924 picture is enlarged the decorative work on the walls and ceilings cane be seen. A solemn re-opening of the Church took place on 20th June 1924.

The monks were conscious that the Church building was not complete. The Sanctuary was built of rubble stone and only meant as a temporary arrangement. The rear view of the buildings prove the point. Looking closely at the Church a wing can be seen high up on the right of the Church at the entrance end. This was the new private Chapel of the monks built in 1884. The previous private chapel became a library. 

By 1936 it was decided to finish the job. Under Fr. Cronan Doyle C.P. the old Altar and Sanctuary were demolished and a new large transept and Sanctuary area were built. The design was that of a Mr. Powell Architect and the work completed by Messrs M. O'Callaghan and Co. Ltd. The first sod was cut 3rd April 1936 and the additions took two years. At the same time a two-story wing for students was built at the rear forming the fourth side of a quadrangle. The Church was re-opened on 13th April 1938. After 79 years the work begun in 1859 was complete.

By 1985 the roof over the old part of the Church was in need of repair. Fr. Brian D'Arcy C.P., the Rector undertook a major restoration and re-roofing that lasted one year. On 19th October 1986 another re-opening and re-dedication of the Church took place. The principal celebrant was Cardinal Tomas O Fiaich DD, Archbishop of Armagh.