First, a citation from Donald Crummey talking about the Yejju and citing Merid Wolde Aregay's important work on 16th century borders and ethnic groups (excuse the use of "Galla" in these sources; they are around 20-30 years old before "Oromo" became universally used and "Galla" universally pejorative):
Even more complex issues arise from a celebrated Galla-Amhara confrontation of the early 1800s, that between Ras Gugsa and Dajazmach Zawde. From about 1800 the main Galla in the chronicles were the Warrashek family from Yajju. Ras Ali I had established this ruling line in the 1780s, and it succeeded in dominating the central Christian highlands down to the 1850s from a base at Dabra Tabor. Merid W. Aregay has convincingly demonstrated that the Yajju in the sixteenth century were a Semitic-speaking group, and that in spite of their displacement during the sixteenth-century upheavals and considerable acculturation toward the Galla with whom they intermarried, they retained to a pronounced degree Semitic speech and, at least among the ruling class, a sharp leaning toward Christianity.42 Yajju traditions today, although recognizing strong Galla influences in their own make-up, do not classify the group as Galla.(43) A variety of factors drew them into this pejorative category.44 In addition to the collective dynastic name, Warrashek, individual names such as Ali and Imam reflected the persistence of Islamic influences in Yajju. Most of the Warrashek rulers personally adhered as firmly to Christianity as had Ali, the dynasty's founder; his chronicler called him a new Constantine. However, by 1800 they had embroiled themselves in Christian sectarian controversies, championing the most radical and deviant of the Orthodox sects.45 Moreover, the ongoing power of the Warrashek drew on the cavalry of the neighboring Wallo, groups of a much less ambivalently Galla nature. Finally, the Yajju were in the specially opprobrious position of having established the most successful dynasty of the period, one which manifestly consummated the end of imperial rule. This accumulation of offenses was enough to have their Galla heritage turned against them.
And now, Merid Wolde Aregay's thesis itself, followed by excerpts from the Futuḥ :
The Muslim peoples under Ahmad comprised, as we have seen, nomads, pastoralists and agriculturalists. It appears that much of the sedentary population had moved into the nearby highland provinces leaving to the other two sections some space in which to spread themselves. The case of El-Ijju [العجو = al-`ijju. First vowel is unmarked so uncertain, but it's unlikely to be `ujju or `ajju] is an indication of such a movement. The El-Ijju lived in a district of Ifat called Qawat (ቀዋት), a land so fertile, says Chihab ed-Din, that is was known as a smaller Gojam. They were Christians at the beginning of the conquest and had a language distinct from the languages of the neighbouring Amhara and Muslims. Though a large part of the El-Ijju embraced Islam, their ruler and some of the people remained Christians. Many El-Ijju men were made to accompany the Imam in his conquest of Amhara.1
How or when the El-Ijju of Qawat came to settle in Angot is not known but there can be little doubt that they are the forefathers of the Yajju of Wallo. Had the ancestors of the Yajju been soldiers of El-Ijju stationed in Angot, it is unlikely that they would have survived the vengeance of the local Christians or the later pressure of the Galla. The El-Ijju must have moved to Angot in large numbers and the reason they did not return to their fertile ountry after the collapse of Ahmad's empire must be because Muslims from across the Awash had occupied Qawat. The Yajju trace their origin to a Sheikh Umar who settled in Angot in the time of Ahmad.1 The Galla, who have failed to subdue or assimilate them completely, know them by the name of Warra Sheikh.2 The Yajju speak Amharic and, as their history shows, have adapted themselves more readily and rapidly than the neighbouring Marawa, Wechale or Wallo Galla to the traditional social and political structures of Christian Ethiopia.
They had put on, had dressed themselves in Christian apparel. The imam said to them, 'By God, do not speak except in the Yejju(552) language.'
The storyteller, may God have mercy upon him, says: While they were on their way, a Christian woman came, crying out to the imam, thinking him to be the patrician Degalhan. She drew near to the imam, and he sought to speak to her in the Yejju language. But he spoke in the language of the Muslims,(553) saying 'give back her possessions.' She understood his language, fell back towards the rear and sat down, saying, 'These are Muslims,' but no one paid any attention to her.
552. العجو: sic! seems to be the name of a district. See infra p.292 where Beshara leaves with 'soldiers of `Ijju'. B: 'en langue idjdjou (barbare?).' (S) Possibly a reference to the language of Yejju, a locality in Wallo, or, perhaps more generally, to Amharic, a tongue which is nowhere specifically mentioned in the Futuh. Yejju today speaks a regional dialect of Amharic significantly different from that in most other areas. Basset's suggestion (Histoire de la copnquête, p. 291) that the passage refers to an Agaw language would seem improbable in that there is no evidence of a presence of Agaws in the Yejju area. (P)
553. Presumably a reference to the Adaré, or Harari, language. (P)
The Qawat people, a part of the people of Yejju,(664) who had converted to Islam said, 'We are Muslims, and have been guarding our region. If one of the Christians came to us, we would kill him, waiting for you to reach us.' The imam was delighted and clad their leaders in robes of honour. The region of Qawat had been under the command of Khalid al-Waradi. The imam had placed them under his charge because he was responsible for their becoming Muslims. Since he(665) perished of the plague, Beshara was put in charge of them. Qawat is a delightful district that the people of Abyssinia call Lesser Gojjam because of its plentiful blessings. Beshara set of with soldiers from Yejju, for their country, and remained there.
Shewa Robit - the main town of Kewet/Qawat woreda w/24,000 people (houses not really pictured); a "Lesser Gojjam" indeed.
There's a lot of info to digest there. Qawat seems to have been both a district and a people. At once part of the Yejju and their homeland. Perhaps the Yejju were primarily in Qawat and the Qawat people were its largest tribe, while there were Yejjus living in bordering regions. Either way, Yejju is clearly differentiated from the language of the Muslims and would seem to fall in the same group of languages as Argobba and the Ifat language (another South Ethio-Semitic language lost to the pages of time) that must have in medieval times formed a continuum from Shewa and Amhara (where Amharic was spoken) to Harer, where Harari is still spoken (this theory has been proposed by, e.g., Grover Hudson, albeit with no reference to the Yejju). The Qawat people seem to be recently converted and other Yejjus are Christian, so it would follow that the Yejju were, prior to the 16th century, a wholly or predominantly Christian group. This makes sense with their original location in Qawat, which is in the highlands of Shewa. It's also not surprising that they converted considering the significant Muslim influence they must have been under. The Ifat, who were Muslim, lived just to their east on the Eastern slopes: still in the highlands, but gradually descending into the lowlands to the East; his was also the general location of the Sultanate of Shewa (which has recently had some very interesting finds uncovered: http://www.culturekiosque.com/art/news/ethiopia.html). We still have very little data on these groups, however. Studying these small, almost disappeared groups and the products of their ancestors will hopefully be a focus of researchers in the future, before it's too late and they have disappeared altogether.