The Grey Seal is bigger than the Common Seal. The Common Seal has a dog like face. The Grey Seal has a roman or prominent nose. It is not difficult to tell them apart. The Common seal is found in estuaries and bays, especially where you find sandbanks exposed at low tide. All seals like to bask in the sun at low tide. The Grey Seal is more common on exposed coasts and favour remote islands to breed. The Common Seal have their young in late May early June. The Grey Seal have their pups late September to October on lonely seashores.
On the 4th May I visited Westport Bay and was surprised to see few Common Seals hauled out on stony reefs at low tide. Perhaps they disperse in spring when food is scarce. I expect the numbers will slowly build up as the breeding season gets underway.