Eleven subspecies of the White Wagtail (Motacilla alba) are currently recognised and they all have slightly different plumages.
This is Motacilla alba yarrellii (in Dutch: Rouwkwikstaart), a subspecies that occurs in Britain.
It has a much blacker back than the nominate race Motacilla alba alba and the black of the throat continues on the side of the neck.
1st, 2nd & 3rd photo: A male
4th & 5th photo: A juvenile
Glendalough Monastic Site area, Wicklow Mountains National Park, Ireland, 9 June 2018