Objects conveying invisibility are relatively common in European folklore. These are often identified as caps or cloaks of invisibility (Tolkien used the motif for the elfin cloaks that hid the hobbits in moments of crisis). Stith Thompson in his Motif-Index of Folk-Literature (1955-1958), provides examples of these things. See, in particular, his "D 1360 Magic object effects temporary change in person": under this motif category are a number of examples of objects that grant invisibility including the sources where these can be found. These include magic stones, seeds, flowers, the heart of an unborn child, magic feather, magic herb, magic cloak (for which there are extensive examples of this popular motif), saint's tunic, magic belt, magic hat/cap, a saint's cowl, a magic ring (again, Tolkien!), magic jewel, magic helmet, medicine, salve, charm, drops, wand/staff, mirror, mask, song, animal's heart, tiger's hair, shirt, and boots.

You get the point - lots of possibilities. The example you specifically are asking for is described by Thompson as D1361.18 "Magic sword renders invisible (Cf. D1081)," for which he gives one source: *Chauvin VI 66 No. 233, referring to a French-language source (for an online version of Thompson, go to this ).

The specific source that Thompson identifies does not matter as much as the widespread nature of the idea - his source is likely not at all related to Carnwennan. Rather, the range of possibilities underscores how the folk mind works, and how easily it was to conceive of an object that would confer invisibility upon its owner. I cannot speak to the specific history of your example or how it worked its way into the Arthurian mythos as expressed in the Welsh Triads; I could not find evidence of anyone addressing this specific motif (but that doesn't mean that it hasn't been tackled by someone!). On a generic level, I can say that this motif is not startling - it is, in fact part of a commonly-used spectrum of possibilities in oral tradition. In addition, it is not surprising that the folk motif "bled into" Arthurian literature at some point.

I hope that helps (I see you posted this a year ago without satisfaction!).

edited to remove clunky language, hidden thanks to a virtual dagger of invisibility.