Final answer:
The Mersey River flows through Manchester and Liverpool before emptying into the Irish Sea.
Step-by-step explanation:
The river that flows through Manchester and Liverpool, before emptying into the Irish Sea, is the Mersey River. This river is not only significant for its role in commerce and transportation in the region but also as a cultural and historical symbol for the cities it connects. The Mersey River starts in Stockport, Greater Manchester, and flows westwards towards Liverpool, where it forms an estuary and empties into the Irish Sea. The cities are located on opposite sides of the river, and the Mersey Estuary is a key geographical feature in the region.