We have to be careful with the term Somali Kenyan . You have three distinct groups :
1. The Southern-most Somali or Somalioid clans who lived in and amongst the Borana for centuries . It was common for them to practice the system of sheegato ( client clan alliance ) depending on who had the upper hand . Obviously these groups intermixed with Boranas by way of the client structure and by conquest and enslavement of oromo captives . These groups will no doubt have more omotic and Oromo-like DNA than Somalis to the north .
2. The second group are the Darood clans who came later and were very successful in driving out the Wardai and Borana from the NFD region . Group 1 and 2 were keen on expanding to central and Western Kenya in the early 20th century . It was only the British colonial interference which prevented a mighty showdown between the Somali clans and the Masai, Samburu, Kikuyu, etc .I am not sure how much omotic or Oromo traces this group ( Marehan and Ogaden ) will have .
3. FOBS or Flight 3 from every single Somali clan who sought refuge in Kenya after the civil war . This group probably has little Oromo DNA.
There is a fourth group who are mainly Isaac and Harti settlers. They have intermarried with different Kenyan and Tanzania clans , but are too small to have affected the overall genetic make up of Somalis there .
Suugo!