Not really a black-headed bird, more chocolate-brown - in fact, for much of the year, it has a white head. It is most definitely not a 'seagull' and is found commonly almost anywhere inland. Black-headed gulls are sociable, quarrelsome, noisy birds, usually seen in small groups or flocks, often gathering into larger parties where there is plenty of food, or when they are roosting.
The pomarine skua is a large seabird, nearly as big as a herring gull. It has long spoon-shaped tail streamers. There are two colour forms: dark - birds are all dark brown with small white flashes on the wings; light - birds have a pale breast and a dark cap on their heads. It does not breed in Ireland, but is a passage migrant in both spring and autumn. It winters off the coast of West Africa.
from Ronan McLaughlin's descriptions of Irish Seabirds