Loch Lomond would have been near the administrative heart of the remnants of the British "Old North" - that area, originally stretching from north of Glasgow to the Peak District, which (outside of Wales) the British held onto longest after the Roman retreat. The Leven, which runs out of the loch, runs straight past Dumbarton Rock (Dùn Breatainn: Castle of the Britons: Info), which the Harleian genealogies (See Other Works) suggest was the chief seat of the kingdom of Alt Clut (Info). Indeed, the Clach nam Breatann "The Stone of the Britons" at the northern end of the Loch probably marked the most northern extent of the post-Roman kingdom, with the Irish-influenced Dalriada (Info) to the northwest and Pictish tribes (Info) to the east (though note that there are a few British placenames further north*).
The kingdom was probably still British at the time our list was initially compiled into the Historia (c.829 CE*). The Brut Y Tywysogyon versions record Alt Clut being destroyed by Pagans (Vikings) in 871 [870 in Manuscript]*, though there may have been Angles in the area as well as Britons (see Annals of Ulster U871**), while the Annals of Ulster record Artgal, king of the Britons of Strathclyde, being killed in U872, and Mael Coluim son of Domnall, king of the northern Britons in U997* While the wonders that follow are largely in South Wales, this one remnant from the north may have been close enough to a well-known British population centre to warrant its recall and placement at the top of the list.
The name Lumonoy was suggested by Watson in The History of the Celtic Place-Names of Scotland as coming from the Early Welsh lumon : "Beacon":
In Nennius, Loch Lomond is "stagnum Lumonoy"; the chapter headings have "De magno lacu Lummonu, qui Anglice vocatur Lochleuen in regione Pictorum"; "Of the great lake Lummonu, which is called in English Loch Leven, in the region of the Picts". The name in modern Welsh form would be Llyn Llumonwy; it is from llumon, a chimney, a beacon, as in Pumlumon, Plynlymon. The tale of Kulhwch arid Olwen recounts how "Kai and Bedwyr sat on a beacon cairn on the summit of Plynlymon". The ending -oy, later -wy, is that in Cornwy, later Kernwy (sic), Cornwall, from Cornavia, "horn-land". Thus Lomond (llumon) is primarily the beacon hill, Ben Lomond; Lumonwy was the district at its base. The Lomond Hills in Fife are, of course, also "beacons", and one has only to look at the peaks of the East and West Lomond to see how well they were suited for that purpose*.
Llumon is presumed to derive from laom "blaze"*, and wherefrom the modern Welsh llumon "chimney".
The "chapter headings" noted by Watkins, however, are not necessarily part of the original Historia. They are a set of summaries attached first to Manuscript L and therefore don't appear in the Historia until the 13th C. They actually have:
De magno lacu Lummonu, qui Anglice vocatur Lochleuen in regione Pictorum et de CCC XL insulis in eo positis, in quibus homines habitant et totidem rupibus, quibus ambitur et de CCC XL nidis aquilarum in eis locatis tantisque fluminibus in lacu currentibus et quod nisi unum flumen fluit ad mare, quod vocatur Leven [Concerning the large lake Lummonu, which in English is called Loch Leuen in the area of the Picts and of the 340 islands in that place, among which men dwell and as many cliffs/rocks, which surround it, and of the 340 eagles' nests on them placed; the great size of the rivers flowing into the lake, also which except one emanates to the sea, which is called Leven]
This is the longest of the summaries, and is chiefly notable for two features. The first is that the lake is "in the region of the Picts", placing it after the earliest Historia manuscripts but with little to date it more firmly, and the second is that the LX rocks of the original is now CCC XL, a difficult change to make through transcription errors alone.
Despite this firm but late indication, there is little to connect the original description to Scotland. This problem is exacerbated by the fact that there actually appears to be no record of the use of the name "Loch Lomond" in sources earlier than the 14th C.* (outside of this rough similarity), leaving an inconvenient gap between the Historia and the current name, and possibly suggesting an alternative location could be sought (Note). Previously evidence points to the Lake being known as Loch Leven*, as suggested in the summary.
Watson suggested the name Levin derived from the Middle Irish for "Elm River"* Leamhain, however this seems extremely unlikely given the breadth of use of the name geographically and the wide use of river names starting l(a/e)v(a/e)n.