The oldest part of the church as it stands is medieval dating from the 12th century, and believed to have been home to a small monastic settlement.
However, there is no mention of a church at St Doulough's in the papal taxation lists of the early 14th century.
It seems likely from all this that construction of the present building started soon after the granting of the site to Holy Trinity by the Pope, and that additions and alterations were made in succeeding centuries, the dates of which are guesswork based on various narratives.
The majority Roman Catholic population was obliged to conduct its services elsewhere, often in private accommodations.
Since 1974, the church of the Holy Trinity in Donaghmede has served the Roman Catholic parishioners of that area, including later-built Clongriffin, and Balgriffin.
A later anchorite, resident in St Douloughs, Eustace Roche, was also a confessor and the record of 1406 states that indulgences were granted to those who confessed to him and made a donation to the church.
Dr Ledwich, in Antiquities of Ireland, is quoted in an 1833 edition of the Dublin Penny Journal to the effect that the church was founded by St Olave, a Norseman, and that this name was corrupted to Doulough.
Dr Ledwich pointed out that the framing of some windows suggests that they were made by assorted carved stones gathered (possibly pillaged, which is what Norsemen were said to be prone to) from various sites.
However, Dr Ledwich's views of the founding saint and the dating of the church have been disproved by recent scholarship, especially by Reeves, Moss and Harbison.
The investigation by Grassroots Archaeology centred on a spot to the west of the church, by the road entrance where there was a ditch in medieval times.
Evidence of the church form the 10th century, was uncovered during recent excavations some 400meters from the village, in place called churchfields.
The complex at St Doulagh's includes, in the field beyond the church and slightly nearer the road, a sunken stone enclosure.
The frescos were damaged by soldiers of Sir Richard Bulkeley of Dunlavin after the Battle of the Boyne.
Down a short path beyond the sunken enclosure is a stairway down to a small rectangular stone building which contains a pool of water, called St Catherine's Pond.
It was common in medieval Ireland for such a room to be added to the west of a church for an anchorite who might be an unordained person following monastic rules.
(O’Keefe, 2015) It seems possible that the building of the tower house took place after the 1406 granting of an indulgence, centred on the church, generated the necessary funds.
The Norman method of operating suggests that they established at some time, at their convenience, their own incumbent priest in St Doulough's.
The tower house could well have been built for such a person, to provide better accommodation and to project a sense of power.
The building of the tower house might suggest consolidation rather than significant expansion since a rural population, possibly poor, was unlikely to expand.
An investigation by Leo Swan in 1987 found evidence of a late medieval burial in the north wall.
In visiting the building that year, Archbishop Burkeley found that the church was in a ruinous state with no ornaments and that the parishioners and indeed the owner of the lands were Catholic and worshipped elsewhere.
(R Moss, 2002) The Civil Survey of Dublin, done between 1654 and 1656 found that ‘the church lands of St Dowlagh’s’ consisted of three small thatched houses and the wall of a decayed chapel.
(Moss, 2002) A stairway was inserted in the south west corner of the Oratory which blocked the lavatory which had earlier been installed in the tower house.
In the medieval period, liturgical practices according to the Roman rite had become standardised in the Christian world, with some regional variations.
A tradition in relation to baptisms may have been in place before any regulations were established that led eventually to the building of a baptistry outside, regarded as the only one of its kind in Ireland.
The Fourth Lateran Council in 1215 decreed the doctrine of Transubstantiation which reinforced the veneration and ceremonial centred on the consecrated host.
There is a taller window with a pointed arch behind the altar position on the east wall, possibly dating from the same time.
A very narrow splayed window by the southeast corner, beside the altar position is said to have been a viewing hole for lepers, who were not allowed in the church.
In the west wall are evidence of blocked-up arches, said to have led at one stage to an aisle, now gone, which was built to accommodate congregation overflow.
Excavations by Leo Swan uncovered evidence of a burial under the north wall, near the altar position.