Generally, I find there's a huge drought of available works when it comes to analysis of Irish Kingship, particularly when talking about the evolution of the concept beyond 1169.
At it's core I want to know if Irish Kings generally wore crowns, and if they did, do we've any idea what they looked like. By extension of this, I'm curious if we've any idea about how this practice evolved? Like would some of the last Gaelic petty kings of the 15th and 16th century wear crowns?
Any reading recommendations on this general topic would also be greatly appreciated
Sorry about the late answer.
Generally, the answer is no. Pre-Norman Irish kings are never depicted wearing crowns in sculpture or art (in fact, I can't find any early depictions of male headgear), nor is there any mention of them in texts. The only depictions of a crowned king I'm aware of in pre-Norman art is on the Romanesque frescoes of Cormac's Chapel, but these are clearly derived from European artistic models rather than Irish dress. Likewise, Norman-era depictions of crowns in Gaelic contexts seem to be derived from English models of kingship rather than local models.
Rather, the mark of Gaelic kingship in the early medieval period was the wearing of purple and of gold or silver brooches, both practices derived from late Roman fashion. These brooches are the closest equivalent to regalia in the early period, with law texts restricting the use of gold and gem-encrusted ones to provincial kings and those of silver to subject kings. They seem to have been passed down through the generations as dynastic heirlooms. There are several sculptural depictions of these crownless kings with prominent brooches from the 9th-10th centuries, such as at White Island, on the Cross of the Scriptures and on Muiredach's Cross (where Jesus is depicted in kingly attire). Generally these fell out of use after the tenth century though.
Neither were there any coronations. Inauguration ceremonies took a variety of forms ultimately derived from the pre-Christian 'feis' in which the king was ritually married to his kingdom. These took place at ancestral royal sites like Tara and Tullaghoge and might involve placing a foot on an inauguration stone, receiving a 'rod of kingship' or, in the late medieval period, sitting upon a stone throne, but they did not involve a crown. One such inauguration is depicted in a rough Elizabethan sketch.
That said, while true 'crowns' never seem to have been used by the Irish, there are depictions of a kind of conical headress worn by Irish cheiftains in the 16th and 17th century. These are depicted in John Derricke's 1581 Image of Irelande (see here, here and here, for example) and again a century later in a 1680 portrait of Sir Neil O'Neill in traditional dress. I'm afraid I can't say much about these or when they originated, since I've only really studied early medieval dress.
There is also an Iron Age artefact that's usually described as a headdress, the so-called 'Petrie Crown', as well as a now lost sheet-gold cap (or possibly a bowl) that was found in 17th century Tipperary, but nothing like these are known from the historic period and their usage is unknown.
Sources: Elizabeth FitzPatrick (2004), Royal Inauguration in Gaelic Ireland C. 1100-1600: A Cultural Landscape Study Niamh Whitfield (2001), The ‘Tara’ Brooch: an Irish Emblem of Status in its European Context, in From Ireland Coming: Irish Art from the Early Christian to the Late Gothic Period and Its European Context
