The Kilmainham Brooch is a late 8th- or early 9th-century Celtic brooch of the "penannular" type (i.e. its ring does not fully close or is incomplete). With a diameter of 9.67 cm, it is a relatively large example, and is made from silver, gold and glass, with filigree and interlace decorations. Like other high-quality brooches of its class, it was probably intended to fasten copes and other vestments rather than for everyday wear, as its precious metal content would have made it a status symbol for its owner; less expensive Viking-style brooches were typically worn in pairs on women's clothing.

The brooch is dated to the late 8th or early 9th century based on its 8th-century design patterns, along with the fact that silver was not easily available in Ireland until the first Viking invasion in AD 795. It was found in the 18th century during an excavation of a Viking burial place in Kilmainham, on the fringe of the city of Dublin in Ireland. Although established as being of Irish origin, its form and decorations seem heavily influenced by both Viking art (in material) and earlier Pictish metalwork (in design and technique). It is held at the Kildare Street, Dublin, branch of the National Museum of Ireland, where it is on permanent display in the "Treasury room".

The brooch is classed as penannular as its ring is incomplete (does not fully close). It is made from gold, silver and (mostly red) glass and enamel, and is similar in form and material to the better-known Tara and Hunterston Broochs.

The frame is made from cast silver and contains cells that once held gold spiral filigree and glass inserts, although some of these are now lost. The ring is outlined by double ridges in high relief, with flat areas reserved for decorative elements. Its top contains a wide oval compartment or cell bearing traces of red enamel, while the sides are formed from gold plates bounded by twisted wires and interlace decorations. The ring ends on two quadrilobate terminals (i.e., consisting of four lobes each), which are formed from a central square surrounded by semi-circular or crescent lobes, three of which are free, while the fourth is shaped and attached to the arm of the ring itself. The format of the quatrefoil terminals has been compared to the opening folio for the Gospel of Luke in the c. AD 800 Book of Kells. The overall head is strap-like when viewed in cross-section.

The tracery on the curves of the ring is made up of interlaced bodies and legs of zoomorphic animals, including an Irish elk. Most of the original pin head is lost, with only its base remaining intact, and has traces of red enamel. The reverse is mostly flat but repeats the shape of the oval compartment on the front. It has some interlace designs, including two interlocking dogs.

The brooch's style has been described as of "Ecclesia-Gothio character" with "fine workmanship", and compared to the Tara example although "not so fine". Although described in 1989 as "probably Irish", it contains structural and decorative designs, including its lobed (i.e. "hanging") terminals, small cusps, and the cartouche on the hoop, are reminiscent of the Pictish-syle, indicating that it may be of Northumbrian origin (unlikely), or at least heavily influenced by that style.

Brooches of this type are the most common and thus studied form of surviving Irish and Scottish medieval metalworks due both to their then popularity and inherent durability. Examples such as the Kilmainham Brooch were built by skilled craftsmen and their workshops using precious metal, and intended as status symbols for wealthy (and often female) commissioners.

The Kilmainham Brooch is usually dated to the late 8th or early 9th centuries as it is seen as transitional in both style and material. Its annular form and use of filigree place it in the 8th-century Irish tradition, while its use of silver, as opposed to gilding, indicates at earliest an early 9th-century origin, that is in the period after the 795 AD Viking invasions of Ireland, when silver became more available to native metalworkers.