Recent radiocarbon dates from sites excavated in the Irish midlands suggest that knowledge of ironworking may have been known from as early as the eighth Century BC. At Kinnegad 2, Co. Westmeath, charcoal found associated with iron slag and pottery of Late Bronze Age type, yielded a date range of 810 - 420 BC. At Rossan 6, also in Co. Westmeath, charcoal associated with iron slag yielded a date range of 820 - 780 BC. The transition to the widespread use of iron, in preference to bronze, appears to have happened slowly and over a long period, and it was not until around the third Century BC that a distinctive Iron Age society, clearly recognisable in the archaeological record, emerged. What gives the period its distinctive character is the widespread use of ornament to decorate objects using an art style that was developed first in central Europe by Celtic peoples. Known as the La Tène style, the art occurs on early objects, such as two imported gold collars found in a bog at Ardnaglug, Co. Roscommon that were probably made in the Rhineland in the third Century BC.
Much of the Iron Age collection was uncovered during river drainage schemes and the cutting of peat bogs during the 19th Century and some important finds were acquired without their provenances being recorded. The collection is rich in high quality bronze objects that were found in hoards, together with some highly important hoards of gold artefacts. The native metalwork of the period is distinctive and contains types of objects that appear to be exclusively Irish, such as large ornamental bronze discs of unknown function and elaborate Y-shaped pendants that appear to have been used to lead horses in procession. It appears that the hoard material was deposited mainly as votive offerings placed in bogs, lakes, rivers and along the seashore, and the types of objects represented are seldom, if ever, found in settlement or burial contexts. Usually, the hoarded artefacts fall into a number of clear categories including personal ornaments, weapons and tools, horse furniture and harnesses, and feasting equipment.
Personal ornaments include pins, fibulae, beads, bracelets and neck ornaments. Simple collars of twisted gold strips are known but there is also the sumptuous gold collar found on the seashore at Broighter, Co. Derry. This find was associated with imported neck ornaments of Roman workmanship as well as an exquisite gold model of a boat, complete with mast and fittings, and a model cauldron.
Feasting equipment comprises metal and wooden vessels, including cauldrons, bowls, cups, platters and large wooden troughs. Horse furniture and harnesses include the Y-shaped pendants mentioned earlier, which have been found in association with pairs of bridle bits. Wooden yokes found in bogs and the remains of wheeled vehicles are also included in the collections. Weapons include swords, scabbards, chapes, spearheads and spearbutts. Among the organic finds from bogs are votive deposits of butter, a leather shield and items of clothing fashioned from wool, leather and hide. There are also a number of mummified bodies, referred to as bog bodies, which are the remains of persons who were killed as part of a sacrificial ritual during the Iron Age. There is also a small amount of carved stone including quern-stones and stone heads of which the Corleck Head is a well-known masterpiece.
The collection also contains imported Roman material including objects recovered from archaeological excavations and stray finds. Among the exceptional finds is a hoard of silver ingots and chopped up fine silver tableware from Balline, Co. Limerick. Other finds include coins and jewellery excavated on cult sites at Freestone Hill, Co. Kilkenny and Newgrange, Co. Meath. A number of objects imported from Roman Britain, including weapons and personal items, were discovered in an Iron Age cemetery on Lambay Island, Co. Dublin.