Very interesting page with some intriguing discussions. its been a while really.
Some of the arguments aggressively presented above appear quite well sourced when they're in fact queen Ann at the front, and aunt Sally at the back.
Ahmad Al Ghazi was never described as a Marehan, not once, in the biggest documentation of the epic battles against the Abyssinians; namely Futuh Al Habasha. Now I have been fortunate enough to have had a look at the book in its original Arabic, and I will be posting my findings below with important passages highlighted for future reference.
The first mention of Hiraabu, the leader of the Marehan, being described here as a two faced hypocrite. In this page we read on how the Imam, took part of his army to go and face Hirabu and his [Hirabu's] tribe; notice they didnt say that the Imaam went to face his tribe, but Hirabu's.
It also has a passage where the Imaam goes back with [the tribe of] Marehan. Not [his] Marehan, keeping in mind that if Marehan was really the Imaam's tribe, the writer would've added the Taa' to make it قبيلته المريحان instead of قبيلة المريحان.
Voltage wrote:Imam Ahmed was the leader of the Muslims with his once cousin Hirabu being the cheiftain of the Eastern Somalis
Hiraabu was never the Imaam's cousin. This part tells the arrival of Hiraabu on the Imaam and he is described as "someone called Hiraabu". Not "the Imaam's cousin Hiraabu". If there was any relationship between Hiraabu and the Imaam, the word Shakhs شخص wouldn't have been used since its primerely used to describe a person of ignorable status. Check below:

In here the incident that took place between Gerre and Marehan is told and how the Imaam dealt with it. Marehan is described as "another tribe called Marehan". قبيلة اخرى من المريحان. If it was the Imaam's own tribe, why not simply refer to it as قبيلة الامام المريحان or the Imaam's tribe of Marehan instead of saying "another tribe called Marehan"? Because there was no link between them.
I am by no means taking away from the Marehan here, I am merely correcting some historical tales that seem to have gotten lost in translation, and that doesnt take away from anything the Marehans have achieved in the times of the Imaam. They were referred to throughout the book as the Somali, and I feel that in some way I can be proud of that, of their role in that great war.
/Rampage