Crisps and even chocolate are some of the food being given to the Phoenix Park deer.
Rangers are urging people not to feed or take selfies with the animals while out on their Christmas walks.
More than 500 wild deer roam the Phoenix Park.
Terry Moore, head ranger and deer keeper, says messing with their diet can lead to malnourishment and even leave the males with smaller antlers.
People in the park are being told to keep 50 metres away.
Instead of feeding and taking pictures with the deer - post boxes for children to drop off their letters to Rudolf have been installed.
Reporting by Emma Tyrrell
