The surviving Harla among the Issa clan that remains that Enrico Cerruli and many other 20th century writers have had the privilege of studying claim darood. They don't claim Dir lineage.
The Ruined Towns of Somaliland* - Volume 11 Issue 43
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And
Book Source: Digital Library of India Item 2015.81470dc.contributor.author: Ralph E Drake Brockmandc.date.accessioned: 2015-06-30T17:10:47Zdc.date.available:...
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If Harla was assimilated by Darood why are they part of Issa while claiming a separate lineage? and why would the ones in Afar claim the same separate Darod genealogy? Why do they speak their own Somali dialect and not the one spoken by Issa/Darod?
Use your common sense. Futuh groups Sedentary Harla with other Bedouin Somali tribes but also distinguishes them based on lifestyle differences.
There is Tarikh Al Mulikh and she mentions that there are other short texts on Awsa covering later periods. I am assuming its Tarikh Al-Mulikh that speak on the power installation against Somali groups in that area.
I didn't say it mentions explicitly an alliance between Isa and Harla. That's just my own theory. The name ''Harla Isa'' in the genealogy of the later rulers makes that theory seem like a possibility.