As is the case with many languages, re- search into code-switching in Modern Irish has, until recently, mainly been focused on the spoken language. Online user- generated content (UGC) is less restric- tive 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. 1 Introduction User-generated content (UGC) provides an insight into the use of language in an informal setting in a way that previously was not possible. That is to say that in the pre-internet era (where most pub- lished content was curated and edited), text that was available for analysis was not necessarily re- flective of everyday language use. User-generated content, on the other hand, provides a clearer snap- shot of a living language in natural, everyday use. Analysis of minority language UGC in partic- ular provides much insight into the evolution of these languages in the digital age. In some bilin- gual environments, the overwhelming dominance of a majority language can sometimes restrict and discourage the natural use of a minority language. c 2019 The authors. This article is licensed under a Creative Commons 4.0 licence, no derivative works, attribution, CC- BY-ND. Online environments, on the other hand, can offer a kind of ‘safe space’ in which these languages can co-exist and the minority language can thrive. Ad- ditionally, various interesting linguistic phenom- ena occur online that may be frowned upon in more formal settings. The present paper aims to investi- gate one such phenomenon among Irish-speaking users of the micro-blogging platform Twitter. Code-switching occurs whenever a speaker switches between two (or more) languages in a multilingual environment. Negative attitudes towards code-switching have been documented widely in this field – in particular earlier beliefs that code-switching indicated a communicative de- ficiency or lack of mastery of either language. In fact, the phenomenon is now understood to be in- dicative of bilingual proficiency (Grosjean, 2010). Solorio and Liu (2008) note that “when the country has more than one official language, we can find instances of code-switching”. Given that Irish is the first official language of the Republic of Ireland, with English as the second,1 and given the well-known existence of code-switching in the spoken Irish of the Gaeltacht regions (Hickey, 2009), it is unsurprising that Lynn et al. (2015) and Caulfield (2013, p. 208ff) report that code- switching is a common feature in Irish UGC. Our earlier work (Lynn et al., 2015), however, focused only on a part-of-speech (POS) tagging analysis of an Irish Twitter data set, without further ex- ploration of the code-switching phenomenon that was observed. In fact, the English (code-switched) segments of tweets were given a general tag that 1Note that English is the more dominant language, with only 17.4% of the population reporting use of Irish outside the education system http://www.cso.ie/en/media/ csoie/releasespublications/documents/ population/2017/7._The_Irish_language. pdf Proceedings of the Celtic Language Technology Workshop 2019 Dublin, 19–23 Aug