An umbilical hernia is when the muscle wall of abdomen doesn't close, and a small bubble like protrusion under the skin can be can be seen and felt, as a soft lump on the tummy. Some may extend into the diaphragm.
Some soft hernias will just seal off as puppy grows, and again we take all hernias under advisement from our vet, and a record is made on the vaccination card. If the vet perceives that no further attention is required then we leave the hernia, however if our vet says an operation will be required, our vet undertakes this procedure before a puppy leaves for their new home, and the all clear is given by our vet.
Most umbilical hernias in dogs are inherited, although trauma at birth can also be the cause. I always know if I'm going to end up with a hernia pup through trauma, and despite being mindful of this when whelping, other issues can take priority at the time. Hernias which extend into the diaphragm are more serious and without doubt inherited.
However I am also of the opinion that Irish Wolfhounds have more detrimental inherited health issues to be concerned about than umbilical hernias, but if we ever had a very high percentage of hernias from a litter, I would take precautionary steps not to repeat it.