J.K. Rowling writes that Ilvermorny was a magical school of witchcraft and wizardry, founded by a pureblood Irish wizard named Isolt Sayre in the 17th century. Ilvermorny sat atop the highest peak of Mount Greylock enshrouded by powerful concealment enchantments.
Sayre was a descendant of Salazar Slytherin and the renowned Irish witch Morrigan. Sayre lived with her parents in a small Irish town till the day her mother's estranged sister Gormlaith Gaunt (pureblood zealot) murdered Sayre's parents and kidnapped her away to Hag's Glen to stop her from marrying a muggle.
A decade later, following imprisonment and continued attempts at indoctrinating Sayre, she stole her aunt's wand, and managed an escape.
Sayre went on to pose as a Muggle boy, sailing to the 'New World', where she came across a wealth of magical creatures and fellow wizards that'd ultimately help her create Ilvermorny.
The four Ilvermorny houses are named after magical creatures:
Check out the story over on Pottermore that was just released today!