Ego-documents like personal letters have emerged as an important resource for historical linguistics, as shown by studies of English and other languages. The last decade has also seen a significant increase in the quantity of historical research on Irish English based on evidence from emigrant letters. This research investigates diachronic developments in an important variety of English, expanding our knowledge of the history of Irish English and complementing previous studies, which have largely used evidence from literary texts, travel accounts, etc. The present chapter surveys research in this field and offers a brief account of what such ego-documents reveal about Irish English, using the example of a bundle of letters written by one Irish emigrant in the 1880s. This text is part of the Corpus of Irish English Correspondence, and it contains usage patterns of those features that can reveal a good deal about the language of the writer’s home locality and Irish English in general.