Agro-wells (shallow agricultural wells) serve as the main source of water for irrigation of agricultural crops in dry zone of Sri Lanka. Agro-wells are extensively used by the farmers in Malwathu Oya Cascade-I, located in dry zone without proper assessment of its suitability for irrigation. Hence, this study was conducted to evaluate the quality of groundwater for irrigation in the Malwathu Oya cascade-I. Some selected vital water quality parameters (electrical conductivity, pH, concentration of sodium, potassium, magnesium, calcium, carbonate, bicarbonate, arsenic, lead, cadmium, nitrate nitrogen, ammonium and nitrogen) and depth to groundwater were measured once a month from June 2012 to May 2013 to represent, both pre and post monsoons in 20 selected agro-wells. Salinity, residual sodium carbonate, sodium adsorption ratio and sodium percentage were calculated using measured parameters. Point based inverse distance weighted interpolation method available in GIS was used to produce maps of spatial distribution of water quality parameters. Based on the height of water column at the end of the dry season, 20%, 25%, 30% and 25% of the wells had low, moderate, high and very high groundwater potential for agriculture, respectively. It was observed that almost all the chemical parameters monitored tend to concentrate towards the lower part of the cascade during the pre-monsoon period. As per the US salinity hazard diagram, 5% of the wells had low salinity and low sodium (C1S1) while, 35% of the wells had medium salinity and low sodium hazard (C2S1). These sources can be used for irrigation with most crops on most soils with little danger of exchangeable sodium. Sixty percent (60%) of the water samples had high salinity, low sodium hazard (C3S1) and can be used with adequate drainage and special measures for salinity control.