The Ishmaelites were the progeny of Ishmael, who accompanied Lehi and his family on their journey to the Promised Land. In the eventual separation of the family into two groups—one embracing the righteousness of Nephi and his followers, and the other pursuing rebellion with Laman and Lemuel and their followers—the Ishmaelites blended in with the latter and were absorbed as part of that segment of the population in the Book of Mormon. Tensions between the two divisions continued for generations. Following the visit of the resurrected Savior to the Saints in Bountiful, there was a long period of peace and unity. However, beginning in the year AD 230, a major separation of the people began to take place.
And it came to pass that they who rejected the gospel were called Lamanites, and Lemuelites, and Ishmaelites; and they did not dwindle in unbelief, but they did wilfully rebel against the gospel of Christ; and they did teach their children that they should not believe, even as their fathers, from the beginning, did dwindle.
In modern scripture the Lord refers to the Ishmaelites and their role in the unfolding of historical events in the Book of Mormon.
Nevertheless, my work shall go forth, for inasmuch as the knowledge of a Savior has come unto the world, through the testimony of the Jews, even so shall the knowledge of a Savior come unto my people—
And to the Nephites, and the Jacobites, and the Josephites, and the Zoramites, through the testimony of their fathers—
And this testimony shall come to the knowledge of the Lamanites, and the Lemuelites, and the Ishmaelites, who dwindled in unbelief because of the iniquity of their fathers, whom the Lord has suffered to destroy their brethren the Nephites, because of their iniquities and their abominations.
And for this very purpose are these plates preserved, which contain these records—that the promises of the Lord might be fulfilled, which he made to his people.