Hi Eamon Roche,
Ishmael and Abraham’s servants became under Abraham’s covenant through circumcision. However they didn’t pass on this faith to their descendants as Isaac did. Nor did Esau pass it on to his descendants. One should not equate biologically descended Ishmaelites with the Arabs or Muslims but they are Ishmaelites through them adopting Ishmael as their spiritual father and thus like him they spiritually enter the Covenant with Abraham. Just my opinion and I think this is a topic open to theological speculation.
cheers Br Gilbert Joseph