The Garda and the Irish Society for the Prevention of Cruelty to Animals are investigating the deaths of more than 40 red deer belonging to property developer Mr Phil Monahan.
Tests are being conducted on one car case in a Dublin laboratory to try to establish the cause of the deaths, which appear to have started over the Christmas period.
Gardai would not comment on whether the animals had been poisoned but confirmed they were conducting a full inquiry.
The deer were being farmed at Glyde Farm near Tallanstown, Ardee, Co Louth. A spokesperson for Mr Monahan told RTE: "The loss to the herd is catastrophic. It is all over the herd, all types of animal. The possible explanation is there has been some substance introduced into the feeding. There are some natural substances that will kill them but none of these is present on the deer farm."
He said it appeared the animals had been poisoned but the outcome of the tests was awaited to confirm this. The animals have been buried but there was concern that more could still die.
The ISPCA confirmed it had started an inquiry after a complaint from a member of the public who saw one of the dead deer.
Its Louth inspector, Mr Sean Mellon, said the complaint was received last Monday and within an hour the deer farm had been inspected. "There were animals lying dead when I got there. I am not a vet and we will have to wait until the tests are completed to see what they say about the cause of death."