The comparative effects of reduced irradiance and soil water on alley cropped herbage are poorly understood.  Our objective was to determine effects of irrigation on herbage productivity when tall fescue [Lolium arundinaceum (Schreb.) Darbysh.] was grown in two sites, a meadow and a loblolly pine (Pinus taeda L.) alley, near Booneville, Arkansas.  Three tall fescue varieties with or without the fungal endophyte [Neotyphodium coenophialum (Morgan-Jones & Gams.) Glenn, Bacon, & Hanlin comb. nov.] were space planted in the meadow and pine alley with or without irrigation.  Herbage mass and nutritive value were determined at 8-wk intervals for 2 yr.  Photosynthetically active radiation (PAR) was 17 and 11% of meadow PAR in 2004 and 2005, respectively.  Irrigation compensated for low rainfall and evapotranspiration in the meadow in 2004 and 2005, a dry yr, but not in the pine alley.  Differences among tall fescue-endophyte associations tended to be small and inconsistent compared to differences due to site and irrigation.  Low solar irradiance was a greater constraint to herbage specific leaf weight (SLW), leaf extension rate (LER), tillers plant^-1, mass tiller^-1, mass plant^-1, and total nonstructural carbohydrate (TNC) than low soil water.  Various silvicultural practices could improve penetration of solar irradiance to the alley crop, although competition for soil water might constrain herbage productivity at the crop-tree interface.