Monaghan is rainy year-round but pleasant in late spring and summer, when the drumlin country is at its greenest. June and July are the most reliable months, with long evenings and a fair chance of dry walking weather.
The town comes properly alive at the end of August for the Harvest Time Blues Festival — a weekend of free pub gigs and outdoor stages that pulls in international names and fills every B&B in the county. Book well ahead if that's your interest. Winters are mild but grey; the surrounding lakes look striking under fog.