Onlookers were stunned to see the body of what initially appeared to be a small dolphin eight miles inland on the banks of a Co Down river over the weekend.
The USPCA animal rescue charity responded to a call from a member of the public on Saturday after the remains were spotted on the Newry section of the Clanrye River.
Spokesman for the charity, David Wilson, told the Belfast Telegraph that the carcass was that of a harbour porpoise and the incident was not being regarded as a cause for concern.
"The creature was dead when we responded to the call on Saturday and we believe it had been dead for some time," he said.
"Harbour porpoises are one of the more common cetaceans found in the Irish Sea and, unfortunately, they sometimes end up being stranded."
The harbour porpoise is one of six species of porpoise and is one of the smallest marine mammals.
Mr Wilson said they are relatively common in Irish and UK waters. "The animal presumably came from Carlingford Lough where there is a known population," he explained. "It is impossible to say what happened, but it is likely that it died at sea and has just been washed up. Only one carcass has been discovered so we are not regarding this as a cause for concern."
As its name implies, the harbour porpoise rarely strays from coastal areas or river estuaries. As a result, it is the most commonly observed of marine mammals, usually spotted by enthusiastic whale watchers.
They often venture up rivers and have been seen hundreds of miles from the sea.
Mr Wilson said that because the mammal is believed to have been dead for a considerable time, an examination of its remains would not be necessary.
"A post-mortem examination conducted on any animal that has been dead for more than a few days would most likely prove inconclusive," he commented.
The USPCA said the relevant agency has been notified.