Well known amongst Chiang Mai's expatriate community. This is a big, spacious place, probably a bit bigger than your usually Irish pub/restaurant - perhaps best to consider as having an Irish theme. Excellent little sports bar upstairs for pub sports enthusiasts and some outdoor seating. Good range and beers and spirits in stock.
Guinness comes in at 160 Baht a pint. O'Malley's is easily accessible by public transportation. To get there by taxi ask for Anusarn's Market and it is nearby.