I am a descendent of Johannes Issler, who migrated to Brazil in the mid-1800's. I once had a book in my possession, written by a certain P. Isler, from Alsace, who gave two possible explanations for the name. The book was in French, so what follows is my understanding of what was written there, and is not 100% reliable (but almost). The first explanation is that the name comes from the word Island: it probably means "someone who came from an Island." The second is that "Isler" is related to the word "Eisner," which means Iron in German. So it may mean "someone who works with iron," or the like. The latter matches one of the possible explanations given in this Home-Page.