This article begins with an introduction to language corpora and the approach to linguistic description arising from corpus linguistic approaches to the description of varieties. It stresses the importance of a well-balanced, representative corpus for general language description and demonstrates the benefits of a quantitative as well as a qualitative approach. It introduces the International Corpus of English: Ireland Component (ICE-Ireland) and presents a case study of the present perfect, with particular reference to the HAVE + -ed participle construction, the after perfect, the medial-object perfect, the BE perfect, the extended-now perfect, and the use of present and past tense with perfective meanings. Throughout the case study, the article underlines the importance of quantitative patterns in the data uncovered by a corpus linguistics approach. *