It must be regarded as a matter of considerable regret that the two oldest large structures in Dublin - the cathedrals of St Patrick's and Christ Church - should both have undergone such radical refurbishment in the 19th century that little of the original buildings was permitted to survive.

In the case of Christ Church, by 1870 the entire cathedral had been allowed to fall into such a state of decay that its very survival seemed open to question. On the south side, for example, masonry that collapsed in the 16th and 17th centuries had left the wall a mass of blocked windows and supporting buttresses.

By the time Christ Church cathedral came to be restored, work on overhauling St Patrick's had been completed but at the expense of the latter's historical fabric, of which almost nothing was left. As a result of the outcry over this insensitive treatment, the restoration of Christ Church was therefore handled with somewhat more respect, thanks in part to the employment of an architect for the task. This was the Englishman George Edmund Street who specialised in Gothic design and who worked on the building between 1871 and 1878.

Just as the refurbishment of St Patrick's was paid for by members of the Guinness family, Street's restoration programme at Christ Church was underwritten by the Dublin distiller, Henry Roe, who was effectively bankrupted by his own philanthropy. And while Street gave considerable, and often considerate, attention to the main part of the building, his treatment of the cathedral's oldest section - the crypt - seems to have bordered on indifference. He argued that the space had "no artistic value," its interest being "wholly archaeological".

Admittedly, in this respect, Street's attitude to the crypt merely mirrored that taken during previous centuries. When he first visited the cathedral in 1868, the architect claimed the crypt was so filled with earth and rubbish that an examination of its interior, especially the eastern part, could be undertaken only with great difficulty, access to the side chapels being achieved "by creeping and crawling on all fours, lantern in hand".

Dating from the first decades of the 13th century, with the eastern chapels possibly almost 150 years older, the crypt of Christ Church served as a massive foundation for the cathedral above.

Running to some 55 metres, it extends under virtually the entire length of the upper structure, an arrangement unique among cathedrals in Britain and Ireland, according to Prof Roger Stalley of Trinity College Dublin. It is vaulted in stone throughout, for the most part covered by irregular groin vaults although in some sections there are short sections of barrel vault. Their character varies considerably with some parts composed of loose rubble fill; at the west end of the crypt, the curves of the vaults change because here Street flattened their crowns during his restoration in order to lower the floor level of the cathedral above.

Within the crypt itself, however, he installed a concrete floor. Until this year, nothing was done to improve on Street's work in the cathedral crypt. However, as Christ Church entered the new millennium, its board made a decision to embark on a thorough overhaul of the building's lower level, working with architect Paul Arnold.

One of the major tasks was to replace the concrete floor with something better - in this case Liscannor flagstones. These now cover the entire space, but have been set slightly clear of the bases of all pillars to leave them free-standing.

Electric spotlights have been set into the stone at many pillar bases to provide atmospheric lighting which may be made stronger or weaker according to requirements. At certain points along the ceiling, small spotlights have also been provided.

Inevitably, given its position, the crypt receives almost no natural light, although there are a handful of small windows at the east end. Street had discovered evidence of the 12th century originals of these when he first examined the space, observing that "as several of the windows remained almost perfect, we were able to ascertain enough to ensure an accurate restoration of its external outline." The windows were reinstated but not, it would seem, with any original cut-stone work being used for the surrounds.

While the new flooring is by no means the only change effected, in certain respects it is the most important as the stone has changed the crypt's character, not least in suggesting the space is worthy of respect as a distinguished piece of architecture.

The smoothness of the flagstones also provides an excellent contrast to the uncut stones used for most of the walls and vaulting, although in a number of places the vaults have been rendered. While clearing out debris from the crypt, Street had continued to regard it primarily as a convenient storage space, and installed various remnants of the cathedral above for which he had no further use; against the north choir wall, for example, he rebuilt the hood moulding of the nave windows in order to demonstrate how accurate his own restoration had been.

Street also moved a series of funerary monuments down to the crypt from the the southern nave. These, however, were poorly displayed and one of the tasks undertaken by Paul Arnold was their reordering along the walls. Regrettably, the largest of the monuments - to a former Lord Chancellor of Ireland, Lord Bowes, who died in 1767 - remains in two sections, being too tall to be shown as a single unit, but others have been sensitively and elegantly placed around the crypt. Among the finest are those to an 18th century Bishop of Meath, Dr Welbore Ellis and his wife Diana, seven of whose eight children are commemorated as dying before they reached their second birthday, and a life-size neo-classical recumbent sculpture of an early 19th century Cavan MP Nathaniel Sneyd.

Other items on display include: a 17th century set of wooden stocks; carved figures of Kings Charles I and II, along with the royal arms of the Stuarts, all of which used to stand above the portico of Dublin's Tholsel prior to its demolition in 1806; and a series of medieval grave slabs as well as a medieval tomb.

It is obvious that, because he had no great aesthetic interest in the crypt and its history, Street probably destroyed a large amount of archaeological material in the course of his restoration programme. To a certain extent, he had no choice in the matter; he needed to reconstruct substantial portions of the entire area, particularly its southern sections in which he created an entrance porch and stairs leading to the cathedral. It is impossible to know now what was lost during the 1870s, although next to a column on the north aisle a small portion of what seems to have been stone steps were uncovered during the recent refurbishment; glass laid into the floor allows visitors a view.

Those visitors, of whom Christ Church appears to receive a large number, are to have even more offered to them from next spring when the eastern end of the crypt will exhibit a number of the cathedral's most important treasures. These include the Royal Plate presented to Christ Church by King William III after the Battle of the Boyne as well as a tabernacle and candlesticks used for Roman Catholic services in the cathedral during the reign of William's predecessor James II.

The material will be accommodated in display units designed for the purpose. Newly reincarnated, the crypt of Christ Church Cathedral has the appearance of an elegant and spacious enclosure. Beautiful in its own right and certainly much more than just a convenient storage space below the main building, it is now indisputably one of the most historically significant structures in Dublin.