On the basis of a corpus of Old Irish texts justified and described in the introductory chapter, this monograph considers cases of variation in the expression of clausal subordination: a pair of synonymous light heads, a pair of synonymous temporal conjunctions ‘when’, a pair of paronymous final consecutive markers ‘so that’, as well as the difference between the latter and a marker of purpose subordinate. Each analysis counts with a sufficient number of cases, and deals with descriptive and diachronic issues. This work contributes to the study of the Old Irish syntax and is also of interest for the investigation of clause subordination in both the typological and diachronic perspectives.