The Somali expansion southwards to present day Kenya basically finished by the 18th century, had there been no European colonization of Kenya and Tanganyka, I'm certain Somalis would have reached Mount Kilimanjaro by the mid 19th century, and have completely taken over the Masai Mara area bordering Kenya/Tanzania.
Contacts with Nilotic groups like the Samburu, Turkana and Masai have only occured recently, and even then, the mixing has been kept to a minimum. This is not the case for other Cushitic speakers, like the Rendille and Borana Oromo, who have also been culturally influenced by Nilotes.