As is the case with many languages, research into code-switching in Modern Irish has, until recently, mainly been focused on the spoken language. Online usergenerated content (UGC) is less restrictive than traditional written text, allowing for code-switching, and as such, provides a new platform for text-based research in this field of study. This paper reports on the annotation of (English) code-switching in a corpus of 1496 Irish tweets and provides a computational analysis of the nature of code-switching amongst Irish speaking Twitter users, with a view to providing a basis for future linguistic and socio-linguistic studies.