Hibernia Seu Britannia minor. While Hibernia was the more common term, Britannia minor was also used.

Hibernia Seu Britannia minor

As for the origin of "Britannia", I believe it comes from Pretani, "probably a soldier's nickname for the inhabitants of the island. It means 'the people of the designs'" and refers to the widespread habit of either tattooing or skin painting (cf Picti).