Revenue officers at Dublin Port seized four puppies on Monday when they stopped and questioned two men during routine operations.
The British nationals were travelling to Holyhead but did not have pet passports for the puppies.
Revenue say the dogs were also not microchipped, as is required under the Animal Health and Welfare Act 2013.
The four male puppies - one Dachshund, one Chihuahua and two Pomeranian - were transferred to the care of the DSPCA.
Two of the dogs seized at Dublin Port | Image: Revenue
They are now receiving veterinary attention.
Revenue adds that a follow-up investigation is underway.
Anyone with information about smuggling is asked to contact Revenue's confidential freephone number on 1800-295-295.