The reappearance of the invasive fish species chub on the River Inny in Co Longford has been confirmed, according to Inland Fisheries Ireland.
Chub have the potential to compete with native fishes for food and space and are potentially a carrier of fish diseases and parasites, the IFI said.
The Government agency said it is investigating the extent of the invasion and assessing strategies for eradication and control.
The fish was captured on rod and line at a targeted location identified by IFI staff who recorded potential sightings at several locations.
Chub are non-native in Ireland, and the River Inny is the only Irish river in which they have been recorded.
The species was subject to removal operations between 2006 and 2010 and it was hoped that chub had been eradicated.
Dr Cathal Gallagher, IFI Head of Research, said: "Ireland's rivers are ecologically important ecosystems, which support significant recreational fisheries for native and established fish species.
"Non-native fish species threaten these ecosystems and the game and coarse fisheries that they support - potentially in unforeseen ways - and are thus a cause for concern.
"We would appeal to anglers to protect our fisheries by not moving fish between watercourses for any reason and submit any sightings directly to IFI or through the IFI hotline."
The IFI hotline can be contacted on 1850 34 74 24.
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This evening the President of the Angling Council of Ireland, Martin McEnroe said the only way to keep invasive species out of the country is to impose border controls on plant and fish life coming in.
He said this was now urgent as the Chub would wreak havoc if it gets into the Shannon system in sufficient numbers.
