The Annals of Ulster were compiled in the 1400s, bringing together information from a number of earlier chronicles that have subsequently been lost or destroyed. This makes them difficult to study, since we don't always know what the original source behind an entry was, or which entries were originally part of the same chronicles. For example, under the year 794, there's eight separate entries—seven in Latin and one in Old Irish—meaning at least two separate chronicles got copied into this single entry (manuscript; transcription; translation; print edition).

The entry you're after is 794.7: Uastatio omnium insolarum Britannie a gentilibus. This Latin translates to: 'The devastation of all the Islands of Britain by gentiles.' In medieval Latin, gentiles could be used to indicate any non-Christian group, and it's often used in contexts which suggest that Christians and gentiles lived in separate places. The implication here is that the author only states that the British Isles were raided by non-Christian non-locals.

Gentilibus is often alternatively translated as 'by heathens'. While this is technically right, I worry that it carries a few false implications. For example, we know that not all 'heathens' practiced a single religion, with marked diversity and rapid change even within relatively small regions of Scandinavia (Svanberg 2003). Furthermore, it would be wrong to suggest that these people raided the British Isles because they were heathens, since we know that early medieval rulers were predators, regardless of their religion. The Annals of Ulster record two or possibly three other raids for 794 not linked to heathens (entries 6 and 8, and probably 4). So the chronicler doesn't seem to be calling these raiders out for doing anything that his Christian neighbors wouldn't do, but rather registering his surprise that these particular raiders came from abroad.

But when the Annals of Ulster were being compiled in the 1400s, where did this particular entry come from? I'm not sure there's a definite answer. A lot of times, when chronicles were being compiled or copied, scribes would insert a few things that they already knew and which they hoped would interest their readers. For example, the earliest copies of the Anglo-Saxon Chronicle [represented by MS A] were written in southern England and didn't even take notice of the 793 raid on the northern monastery of Lindisfarne. But a later scribe from northern England composed a lengthy entry to commemorate this event, including stories of famine and fiery dragons [represented by MS E]. He might have made this entry up whole cloth after reading some letters of condolences which the English expat Alcuin had sent to Lindisfarne. Or he might have been working from a northern chronicle that has subsequently been lost.

Similar questions could be brought to bear on the Annals of Ulster. There's plenty of evidence that Scandinavian-based raiders were harassing the British Isles in the 790s, but when exactly did this information first start getting copied into Irish chronicles? With regard to the Annals of Ulster, we can only say that it must have been some time before the 1400s. Moreover, if the copyist who compiled the Annals of Ulster simplified or edited the chronicle entries, we might not even have the original language.

In sum, the Annals of Ulster don't tell us anything specific about the raids ascribed to the year 794. And in general, I'd hesitate to read too much into the Annals of Ulster, although they certainly help add texture to our understanding of the early middle ages when read alongside other texts, such as the letters of Alcuin or the Anglo-Saxon Chronicle. More definitely, late copies of the Anglo-Saxon Chronicle specify that the men behind a small altercation in 787 came from Hordaland in Norway, and the large number of artifacts from the British Isles that got recycled into Norwegian societies further suggests that people from Norway were behind many of the early raids (Aannestad 2015).