Island  lies in the Irish Sea off the coast of north County Dublin in Ireland. It is four kilometres offshore from the headland at Portrane and is the easternmost point of the Republic of Ireland.

Lambay Island is the largest island off the east coast of Ireland and is about 2.5 square kilometres in size, and rises to 127 metres. There are steep cliffs on the northern, eastern, and southern sides of the island, with a more low-lying western shore. The geology is dominated by igneous rocks, with shales and limestones.

The island was important in the Neolithic period in Ireland as a ground stone axe quarrying and production site. Two outcrops of andesite, or Lambay porphyry as it is more commonly known, were utilised. The quarry site is unusual in Ireland for being the only Neolithic stone axe quarry with evidence for all stages of production, from quarrying to final polishing.
A number of Iron Age burials were discovered on the island in 1927 during works on the island's harbour. The finds included a number of Romano-British items, and the site has been interpreted as evidence for the arrival of a small group of refugees from Brigantia, fleeing the Romans from 71 to 74.

St. Colmcille is said to have established a monastic settlement on Lambay ca. 530 A.D., and Ireland's Viking period began with a raid on this place in 795. 20th century surveying found remains of an enclosure to the south of the present church, and suggestions of a connected moated site (the present church dates from the early 20th century).