The "Save the Boyne" Group uploaded the clip on Facebook in what shows the Navan native wading in on the controversial planning application that would see the installation of a 7.2km pipeline that would pump 400,000 litres of treated wastewater daily into the River Boyne from the Dawn Meats plant in Slane, Co Meath.
Making a passionate plea to protect the historic river on which he "grew up on", Brosnan said: "Water is our most precious natural resource. 70,000 people get their drinking water from the River Boyne. The River Boyne is also one of the most important sites for salmon in Eastern Ireland and home to rare plant species.
"I grew up on the banks of the River Boyne where it meets the Blackwater. It is one of the most beautiful rivers in Ireland and should be nurtured and cultivated as a sentient being of history, beauty and life force of nature.
"As I say, water is our most precious natural resource. Water levels are historically low and our rivers and streams cannot handle excess waste in this time.
"Waste from the Dawn Meats abattoir should not be discharged into the River Boyne. I appeal to the Meath County Council to protect the River Boyne and their community."
Last year Meath County Council granted permission for the pipeline, a decision that drew huge opposition locally with more than 400 submissions made on the plans and an appeal to an Bord Pleanála ongoing.
Residents, local politicians, business owners, angling groups and Slane Castle owner Lord Henry Mountcharles have all spoken out against the plans.
Around 200 people braved the elements walking from Navan to Drogheda in July on the Save the Boyne group's ‘Go with the Flow’ walk to raise awareness of the ongoing appeal to an Bord Pleanála.
Objectors have said recent low water levels have heightened their fears.
They worry there is limited scope for the river to cope with potential contamination with less water flowing during warm spells.
Dawn Meats said it has “a long-term commitment to sustainability and high environmental standards” and takes local concerns very seriously. A spokesperson said its plans will deliver safer water treatment at a higher standard than at present.
“Third-party studies, including a natural impact statement, have assessed no negative impact on the River Boyne or surrounding habitats. The new facility will be subject to regular monitoring by the Environmental Protection Agency in accordance with licensing requirements,” the spokesperson added.
An Bord Pleanála said it has been considering further submissions and its planning inspectorate is due to complete an inspection of the site and a report on the proposals.