July and August are when Galway is at its most energetic, but also most crowded. The Galway International Arts Festival in the second and third weeks of July transforms the city with street performances, gallery installations, theatre, and music. The Galway Races in late July and early August (typically the last week of July) bring the whole of Ireland to town for a week of horse racing, fashion, and outdoor drinking; accommodation is fully booked months in advance and prices triple. Book well ahead if visiting during either event.
September is arguably the best month. The summer visitors have thinned out, the Galway International Oyster Festival runs in the third week, the weather is often still good, and the city settles back into its normal rhythms. October through May the city belongs to the students and locals, the pub sessions are at their best, and prices are reasonable. The weather is wet and the days are short in winter, but Galway in the rain has its own atmosphere.