SOME SAY that shopping has become the new religion and in certain parts of Ireland intensive development has brought retail and religious life face-to-face

That happened in Dún Laoghaire when the Bloomfield shopping centre was being built in the early 1990s on the grounds of a Dominican convent. While most of the order's buildings were being demolished, a group of people (the likes of whom the world must be ever grateful for) fought for the preservation of a tiny Oratory whose simple exterior hid a large and stunning example of Celtic revival art: a testimony to human spirit, drive and creativity.

The Oratory was built in 1919 at the end of the first World War in thanksgiving for peace and also as a war memorial. It was constructed by Louis Monks, who was responsible for a number of buildings in Dun Laoghaire, and he perhaps even designed this one.

The 'decoration' in the simple building was limited to a statue of the Sacred Heart, which was brought over from France, taking centre place amid the pale plaster walls and (then) concrete floor. But now this gold icon has become just one element in an elaborate, multi-coloured, melodic medley of interlacing patterns and multiple symbols, including grinning snakes, and birds whose necks and beaks curl to create Celtic rings.

This gem is enclosed by two buildings and so is akin to the prize found after unwrapping a pass-the-parcel. The original Oratory building has been protected by a larger structure, funded by the EU and designed by the OPW. Surprisingly, but perhaps for good reason, this outer shell does not have vast expanses of glass through which lots of natural light could have entered the stained glass windows in the Oratory.

On entering the outer building you reach a lobby and turn to the right where you meet the white front of the Oratory - its side walls are hidden behind doors.

Open the doors into the chapel and the sight makes you involuntarily draw in breath - every wall is covered in colour and pattern. The room is small- at 5.85m by 3.60m - but as an art work it is huge. It rewards as a complete piece but offers further gains in close scrutiny of sections.

It took 16 years to paint and was the work of Sister Mary Concepta who learnt the art of such illustration from her father Thomas Lynch who ran a company that illuminated manuscripts.

Born Lily Lynch in 1874, the young girl would help in her father's Grafton Street studio during holidays from her school in the Dominican Convent. Their time together was cut short because her father died in 1890, when Lily was just 16, yet she had learnt enough to take over the running of the studio. After six years disaster struck again when the workshop burned down, and Lily returned to the convent becoming Sister Mary Concepta. Here she continued her creative work, teaching art and playing music. When the Oratory was built 30 years later she asked if she could decorate it.

Celtic art had seen a revival, starting in the 18th century and accelerating in the late 19th century and early 20th century as Ireland was breaking away from British rule. In certain parts of the country architecture looked back too - to a time when styles were seen as more representative of Irish culture. The Honan chapel in Cork, for instance, was built in the Hiberno Romanesque style that drew on past designs for inspiration. The 1916 chapel's façade is a copy of the 12th century Romanesque St Cronan church in Roscrea, Tipperary, and its interior is similar to that of Cormac's Chapel, Cashel, Co Tipperary. It even has a miniature Irish round tower at one end.

Much of Honan's interior fittings were hand-crafted, recalling a time when things weren't machine made. This tied in with the Arts and Crafts movement of the early 20th century, which included the likes of William Morris, Dante Gabriel Rossetti and Charles Rennie Mackintosh, the latter also worked in the Art Nouveau style and some Celtic revivalist artists took influence from that art movement while others kept to a more traditional form.

Sister Mary Concepta worked in the latter style - staying true to works such as the eighth century Book of Kells - although the Oratory walls take in influences from Islamic art and also show a good deal of wit in some of the cartoon-like characters.

As with parts of the Honan Chapel, the stained glass windows in the Dún Laoghaire building are thought to be by Harry Clarke - or his studio. He probably catered closely to his client's wishes as they are not in his distinctive style although the eyes are familiar - especially on the window to the right of the statue.

Every pattern on the Oratory walls and ceiling fits so well with the rest that Sister Concepta must have imagined the complete design before she began working - fitting in the task around her other full-time duties in days that began at 5.30am. The work was stencilled on: Sister Concepta would draw the designs on paper and sometimes old blinds and then cut them out.

This allowed her to use mirror images on opposite walls giving a sense of balance throughout. But there is also free-hand work, certainly in the dash and dot design at the top of the wall. Some of the dots were added in by the children as a reward for going to the paint shop for Sister Concepta, who couldn't leave the convent herself.

She used normal household paint but gave very specific instructions as to how it should be mixed to achieve the shades she wanted and those colours are incredible in that they all work together: all the blues, browns, reds, greens, golds, mauves, blacks, whites, oranges and pale pinks combining, in intricate patterns to, paradoxically, create a feeling of calm.

Many of the designs were her own but a depiction of a monster biting a man has been identified as coming from the Book of Kells. Another influence was the Eucharistic Congress in Dublin in 1932, which saw dignitaries of the Catholic church descend on the city: this is recorded on opposite Oratory walls where Sister Concepta has reproduced the Eucharistic Cross, offering a clue as to when she painted this section.

The ceiling is calm, with simple designs and the outlines of interlacing that haven't been coloured in. Sister Concepta had to stop working on the Oratory when she became ill in 1936 and yet the 'unfinished' work has a sense of completion, as if she decided to paint on the simple outlines to continue the overall pattern in the room and make it whole. She died of TB three years later, at the age of 65, but not without leaving this astonishing art work behind as well as linen panels describing her method of working - for her students - which are now in the National Gallery.

• The Oratory is open on Sunday, October 19th from 12 noon to 4pm. It will also be open for certain periods at other times of the year. Contact Heritage Officer Tim Carey on tcarey@DLRCOCO.IE for details