Yes, there is New Medieval Celtic Literature

Thanks to a newly discovered medieval Cornish manuscript fragment Beunans Ke (The Life Of St Ke)
the amount of Medieval Cornish literature has increased by about twenty percent. The fragment was discovered among the papers of the recently deceased Celticist, J. E. Caerwyn Williams, whose papers were donated to the National Library of Wales.

The manuscript, NLW MS 23849D, consists of ten pairs of leaves (20 folios), foliated, 7, [8, 9], 10-13, 16-19, 22-9.

As you can see, there are a number of missing pages. It is written in a secretary hand of the mid sixteenth century, by a professional scribe, copying an earlier manuscript which he refers to as containing five torn leaves. It contains two previously unknown Middle Cornish play fragments. The first play, Beunans Ke (The Life Of St Ke) is about the life of Saint Kea, a Celtic saint favored in Cornwall and in Brittany, and whose life is described in medieval Breton texts.

The second play is possibly even more important, since it is Arthurian in nature, derived from Geoffrey of Monmouth’s Historia Regum Britannice (Books ix.15; x.1-13; xi.1-2). It refers to King Arthur’s quarrel with the fictive Roman emperor, Lucius Hiberius, about British tribute to Rome. It ends with Arthur’s victory and the emperor’s death in battle, and it also alludes to the clandestine relationship of Arthur’s nephew, Modred, with Queen Guenevere (yes, that is one of the Arthurian variants).

I am not aware of any other extant medieval play about Arthur; this discovery is a Big Deal. Neither text is complete, and both are currently being edited by Graham C. G. Thomas and published by the National Library of Wales where he is a Senior Assistant Archivist in the Department of Manuscripts.

Even if you aren’t a medievalist or Celticist, this really is a piece of good news. There isn’t a lot of medieval Cornish literature left; much of it was destroyed during the Protestant Reformation. The last native speaker of Cornish died in the late eighteenth century. There are currently several different forms of Cornish that have been carefully reconstructed as part of a language revival, but no native speakers in the usual sense. Of the extant medieval literature, there are two short pieces, one of them a poem on the Passion, the Pascon Agan Arluth, another a fragment on the back of a charter, offers a somewhat sardonic view of marriage and is the earliest known extant Cornish. The bulk of extant medieval Cornish literature is in the form of miracle and mystery plays from the fifteenth century, specifically a play about the Cornish saint, Saint Meriasek in Peniarth MS. 105. L 430, and parts of a mystery cycle, the Cornish Ordinalia, MS. Bodleian 791. The manuscript contains plays on the Creation, Passion of Christ and Resurrection, in Cornish verse, with Latin stage directions and diagrams. There are also bits of Middle English in the plays, typically in the lines of the devils and other villains.