Loyngarth appears to be the equivalent to the modern Welsh Llwyn garth "Grove/Bush/Scrub Hill/Enclosure" (despite the modern name for the "Mumbles", and its apparent generation (see below), it would be pushing things to suggest this was "Loin Hill"). The earliest identification of Loyngarth as Llwyn garth seems to be by Stephenson*. Hogan*, apparently independently, made the same suggestion for Loingraib, which is given for this wonder in the "Irish Nennius" (the Lebor Bretnach: See Manuscripts and Note on Translation). Neither Stephenson nor Hogan are explicit about this being Oystermouth/Ystumllwynarth ("Llwynarth Bend [Bay?]").
The 1911 Encyclopedia Britannica (for "Oystermouth") notes, with typical circumspection:
The name of the castle appears in the Welsh chronicles as Ystum Llwynarth, which, by the elision of the penultimate, was probably changed by false analogy into Oystermouth — the bay being noted for its oyster beds. Its church is mentioned in the cartulary of Gloucester (1141) as Ostrenuwe... The headland terminates in two rocky islands, which to sailors coming up the channel would appear like the breasts of "mammals," whence the comparatively modern name, The Mumbles, is supposed to be derived.
For reference, the modern Welsh for Oyster is Wystrysen, though equally note the Latin aestuarium ("tidal estuary") which has picked up an initial "O" elsewhere.
Despite the Encyclopedia's efforts it would be good to find a reference to the name Ystum Llwynarth from before the 1800s as it appears as one of the bardic seats in the "Iolo MS." and it is possible the Encyclopedia Britannica entry derives its information on the chronicles from Y Myvyrian Archaiology (See Other Works), and is therefore potentially dubious. The Iolo MS. reference gives Ystum Llwynarth as the base of Urien Rheged, a figure actually based in the northwest of England* (Info), and notes its alternative name as Caer-Gwyroswydd. Conversely, for example, Lewis* gives neither Caer-Gwyroswydd nor Ystum Llwynarth, and states the original name of Oystermouth as Caer Tawy.
It is, of course, possible that Loyngarth had a larger extent that the current Oystermouth, given the modern Welsh name means "Llwynarth Bend". Of course, anyone who has read Caesar's desciption of the Gauls/British worshiping in groves will note the name "Grove Enclosure", but the name could equally be "Grove hill". Carlisle* notes:
the Sea has made great encroachments on this part of Swansea Bay, and a large Wood, called Crows Wood, between Swansea and the Mumbles, which is mentioned in some ancient Records now preserved in the Corporation chest at Swansea, has been destroyed by the Sea; evident vestiges whereof are now to be seen, such as large pieces and roots of Trees, and the sands being intermixed with a black Peat or muddy substance, which are in several places very loose and quick, and dangerous for a stranger to ride over.
This would match the name nicely, but the "Hill" constituent still suggests Mumble's Hill or nearby as the location of the wonder.