The large majority of Medieval Irish glossing is characterised by the alternate use of Latin and Old Irish. In particular, the Würzburg glosses to the Epistles of Paul constitute one of the most interesting collections, from the point of view of the level of linguistic interaction between the two languages in question (in many cases, the linguistic switch takes place within a single clause). It is of course very tempting to describe this phenomenon as a written form of code-switching. This, however, raises a number of interesting questions: are we really dealing here with a behaviour essentially comparable to oral code-switching? If yes, are current explanatory models of code-switching suitable for studying this particular case? And if on the contrary this is not code-switching, could the fluctuation between Latin and Old Irish in written sources be due to other factors, such as borrowing or textual transmission? This paper will address these and other related questions, in the hope of providing new insights on the nature of Early Medieval Irish élite bilingualism.