Mountshannon, Lough Derg is classified as achieving Excellent Water Quality in 2025 based on the assessment of bacteriological results for the period 2022 to 2025. Mountshannon, Lough Derg has achieved an Excellent Water Quality rating for the four consecutive years 2022 to 2025. Annual water quality ratings are generally calculated using monitoring results over a four-year period and are assessed against stringent bacterial limits to protect bather health.

Mountshannon is situated around a series of three small piers to the east and adjacent to the main south-facing harbour. On the shore of Lough Derg, Mountshannon is an important area for wildlife in particular birds and fish species. There is an area of swamp vegetation along the shore. There is a slow and gradual deepening of the water and the mean depth overall of the water in Lough Derg has been estimated to be 7.6m. Activities include bathing, boating, angling and wind-surfing.