'Sweeney: An Intertonguing' is a practice-based creative-critical translation into English, Irish and Scottish Gaelic, in a new paradigm, of the Late Middle and (mainly) Early Modern Irish Gaelic prosimetric text, Buile Shuibhne, in the form of a new and full annotated edition with comprehensive analytical commentary. It goes some way to address the question posed by Flann Mac an tSaoir ‘Cé dhéanfas aon scéal amháin fileata as Fleadh Dhún na nGé, Cath Maighe Rath agus Buile Shuibhne?’ (Mac an tSaoir 1951: 8). It employs a transformative methodology for literary translation, by applying the stratagem of ‘intertonguing’/‘Sweenese’. The method is without serious precedent and may be utilised as an exemplary approach by subsequent researchers and practitioners. In the accompanying commentary, I employ translation and literary theory, to analyse the artistic decisions – auditory, semantic, imagistic and cultural – taken in developing intertonguing/Sweenese. The work develops André Lefevere’s formulation in Translating Poetry: Seven Strategies and a Blueprint (Assen: Van Gorcum, 1977), as refined through the translation theories of subsequent commentators, including Kwame Anthony Appiah, Susan Bassnett, Itamar Even-Zohar, James Holmes, Philip Lewis, Eugene Nida, Maria Tymockzo, Gideon Toury and Lawrence Venuti. Lingua gadelica is also employed in the text as a further innovation. Sweeney: An Intertonguing is presented as an exemplary, useful and unprecedented text, and as a unique substantive intellectual contribution establishing a priority and warranting publication as such. It should contribute to the study of translation theory and practice, Gaelic/Celtic Studies, and creative writing. The beneficiaries of it may include literary and, perhaps, official translators working in any language in what may be considered a new model and an alternative method. It includes a select bibliography, and a glossary of literary, critical and linguistic terms. The referencing system employed is that of Harvard.