Besides potential Sahabah-period proselytization that no one can attribute exact gravity and extent, one comprehensive way of influence would be the northern Somali region had contact with Arabia before Islam through economic motivations. Extensive networks integrated with recurrent social influence had already laid the bridge for Islam to fertilize across the sea once the southern Arabians were Islamized.
Economic interactivity allowed for a cross traversing the oceanic landscape of the ethnic-geographic counterparts, making it possible for Muslims of Arabian origin to settle for extended periods on Somali soil and Somalis living in Arabia, eventually returning as believers.
I would say by the 9th century, a majority, if not all Somalis, were Muslim, with some probably retaining syncretic traditional practices on the periphery but identifying as Muslim (one example is glaring that I will implicitly forgo for the sake of brevity and focus of the thread).
I think the overwhelming Islamic contact spread through intra-logistical networks by Somalis, as I doubt Arabs went much beyond the coastal economic hotspots. This synergy was probably very rapid, harmonizing well with the flexible lifestyle of Somalis with regional economic networks that diffuse culture and social items.
The fact that northern Somalis are not significantly Arabian genetically also explains how there could not have been too many Arabs that settled long-term, moreover, most proselytization likely occurred through Somali intermediaries rather than direct Arab diasporan effect.
Nejashi wuz somali?

seriously though is it likely?p
I doubt it.
Negus, from which the term "Nejashi" derivatively linguistically inovated, is an Ethiopian-Semitic term. If the word was historically used by Somalis to signify a head of the monarchy, that would only mean we were heavily Habashi-fied, which there is no sufficient evidence for.