The River Clydagh is one of the larger tributaries of the renowned Conn/Cullin/Moy system in County Mayo, Ireland. It has its origins in the easternmost reaches of the Nephin Beg Range, rising near Croaghmoyle and flowing east across bog and rugged terrain before joining with the Castlebar and Manulla Rivers. From that confluence, the river travels approximately five kilometers northward to join Lough Cullin, ultimately connecting to Ireland's most productive salmon fishery system — the River Moy. The Clydagh is a characteristically Irish bogland river, meandering through acidic, peaty terrain that gives it a distinctive character. The lower reaches have been dredged in places to reduce flooding on surrounding agricultural land, and the river drains several small loughs along its course, most of which are acidic and hold small trout.