Bretha Nemed Déidenach (the last Bretha Nemed) is one of the two principal surviving remnants of the celebrated Old Irish Bretha Nemed law "school", believed to have being composed early in the eighth century in Munster. The only surviving copy, now part of Trinity College,Dublin Ms 1317 H.2.15B, was transcribed by Dubhaltach MacFhirbhisigh. Another related text, Bretha Nemed Toísech (the first Bretha Nemed) is now British Library MS Nero A 7.
Bretha Nemed Déidenach contains extracts of works concerning poets and bards, along with passages on such subjects as fosterage, sureties, pledge-interests and land law. Much of it is written in the alliterative rosc style in what is now called Archaic or Old Irish. Because of the difficulty of the language, the text has never been translated. In addition, in parts the text is fragmentary.
It is not known where MacFhirbhisigh obtained his exemplar, nor when or where it was transcribed. Another text in the same MS - Duil Laithne - was written at Ballymacegan, County Tipperary, on 5 May 1643, so the Déidenach may have been copied in that period.