Small may sometimes be beautiful, but it is not always terribly noticeable. This helps to explain why the National Gallery of Ireland's collection of almost 500 miniatures has tended to be somewhat overlooked in preference for larger works in the institution's possession. Drawing on its own rich holdings, a new exhibition in the gallery deservedly focuses on the art of the miniature to the exclusion of everything else.

This is unquestionably the correct approach to take when looking at the form because miniature painting was always a separate skill practised by artists who rarely produced anything else. The miniature evolved from illumination decorating medieval manuscripts in gouache and watercolour on vellum; once printing became widespread, the hand-illuminated manuscript declined in popularity and so artists working in this field turned their attention to other areas, such as the production of small devotional images on vellum.

During the 16th century, small individual portraits also began to be painted, usually called "limnings" or "little pictures", and they were intended to be worn as jewellery. Such pictures, oval in shape, would often be set in gold and surrounded by gems.

The National Gallery possesses no such works; the earliest work in the exhibition, a portrait of Sir Anthony Thomas, dates from 1647 and was painted by the highly-successful English miniaturist Samuel Cooper. The first Irish miniature on show is almost half a century later and comes from the hand of Simon Digby, a Church of Ireland clergyman who became Bishop of Limerick and, for the last 30 years of his life, of Elphin. Digby seems to have been prolific in more than just his artistic achievements - he and his wife had eight sons and eight daughters - but his skill as a miniaturist indicates that this could be an important social accomplishment for members of the ruling class. Digby's portrait of another cleric, John Tillotson, Archbishop of Canterbury, was painted in watercolour on vellum, but in the 18th century, other media started to become popular, such as watercolour on ivory.

This was used for a charming pair of miniature portraits produced in 1761 by Dublin-born Thomas Frye who, after moving to London, was one of the founders (and later manager) of the Bow porcelain factory, for which he invented a new material. The two of his works owned by the National Gallery are framed in a gold bracelet setting, indicating that they were intended to be worn as jewellery, in the same manner as the first miniature portraits two centuries earlier.

Ivory portraits could be incorporated into not just bracelets but also lockets, rings and pendants. The nature of the materials used in their creation meant that these miniatures were sensitive to changes of temperature and, more importantly, to damp; for this reason, they were usually encased in glass. Because the pictures were often commissioned to mark an important event, such as an engagement or marriage, their gold frame might be set with precious stones which possessed a symbolic meaning: rubies signified passion; diamonds represented constancy in marriage; and pearls were sacred to Venus, the goddess of love. A variety of other information might also be included, such as cyphers of the sitter's initials, mottoes, a blazon of arms, monograms or personal inscriptions. Among examples of the last of these featured in the National Gallery exhibition is a portrait of Sarah Sophia Banks, sister of the explorer, naturalist and botanist Sir Joseph Banks, painted by Nathaniel Hone the elder in 1768. Presumably created for presentation to an admirer of Banks, the picture carries the following inscription on its backing paper: "A love here fix'd can/very seldom err/Who loves her best is/best Philosopher." Under the circumstances, it is therefore poignant to report that the subject of this work never married.

As this show also demonstrates, miniatures were to become popular as part of formal mourning clothes following the death of a lover or family member. Under these circumstances, the back of the picture would regularly incorporate locks of the deceased's hair. An 1810 self-portrait by Irish artist G. Jackson, for example, has an elaborate reverse containing two locks of his hair from which are suspended a pair of blue glass hearts bordered with seed pearls.

Similarly, a portrait of a man in a blue coat, originally an oval pendant but later adapted as a brooch and painted by Dubliner George Lawrence in 1793, holds on its reverse locks of the sitter's hair arranged in feathered designs interspersed with gold filigree against a blue glass background. These locks take the form of strands of wheat stemming from three seed pearls. Some of the greatest Irish miniaturists flourished during the latter part of the 18th and first decades of the 19th centuries. Among the foremost masters of this period were Horace Hone (a son of Nathaniel the elder) who, although born and raised in London where his father had settled in the late 1740s, came to live in Dublin in 1782 and remained in this country even after being appointed Miniature Painter to the Prince of Wales (later George IV and a great collector of miniatures) in 1795. Among Hone's best-known works now owned by the National Gallery and currently on show is a portrait of the actress Sarah Siddons, painted during her second visit to Dublin in 1784 and subsequently engraved so that this image achieved widespread renown. Hone was also responsible for another iconic Irish portrait: that of Lord Edward Fitzgerald, whom he painted in 1797, the year before Fitzgerald's death following the failure of the United Irishmen's rebellion. Similarly, Kilkenny-born John Comerford's miniature watercolour profile of Robert Emmet, painted by the artist from a sketch made during Emmet's trial in 1803, achieved widespread renown after it was engraved. Adam Buck, from Cork, is a neo-classical miniaturist who deserves to be better appreciated since his work possesses a rare delicacy and grace, exemplified in the National Gallery exhibition by an enchanting watercolour on ivory of a young lady seated with her knees drawn up close to her chin on a sea wall; the pale tones employed mean that Buck has allowed himself to leave certain sections of the ivory entirely untouched by paint. However, despite the remarkable skill of Adam Buck and his successors, by the middle of the 19th century, the art of the miniaturist was in decline.

The reason for this rapid fall from grace was the emergence of photography, which offered similarly small intimate portraits at considerably cheaper prices. Today, many people carry small photographs of children or spouses in their wallets; 200 years ago, the equivalent pictures would have been painted miniatures.

Treasures to Hold: Irish and English Miniatures 1650-1850 continues at the National Gallery of Ireland until December 10th. An accompanying catalogue written by Dr Paul Caffrey, who has curated the exhibition, is available at the gallery priced at £19.99 (paperback) or £40 (hardcover).