The Early Modern Irish recension of the tale relating Cú Chulainn’s death, Oidheadh Con Culainn, has received comparatively little scholarly attention, especially compared with its Early Irish counterpart, Aided Con Culainn. Consequently, little is known about the textual transmission and manuscript tradition of the Early Modern Irish tale. The present thesis seeks to rectify this and give a more accurate view and preliminary analysis of the extant manuscripts, concentrating on the manuscripts that date to before the 19th century. A core element of this thesis is a draft catalogue of these pre-19th-century manuscripts. Taking advantage of the tale’s prosimetric structure, it will be argued and demonstrated that it is possible to classify the manuscripts of Oidheadh Con Culainn into distinct groups. Within the extant manuscripts preserving the tale we can identify a number of versions of it, differing most notably in the poetry that they contain. The classification of the manuscripts into groups can be established on the basis of the poetry that a version of the tale contains; the emerging groups thus established can be used to comment on the transmission of the tale. In order to corroborate the argument for the manuscript groups, we will explore a number of aspects of the text and the manuscripts, such as textual comparisons on both intra- and inter-group levels, possible relations (e.g. geographical) of the scribes, linguistic and metrical variations, the ‘rhetorics’, and different versions of the tale written by the same scribe. The thesis will further investigate the most famous poem from the text, Laoidh na gCeann (‘The Lay of the Heads’), in order to establish to what extent the evidence from the poem can be used to add to our understanding of the transmission of the overall tale.