There are a number of different issues involved here:

(1) Copyright. This exists on the song for IIRC 70 years after the death of the composer. You could use this aggressively and get the Youtube performances taken down or attributed, but it may be better to save your energy for when its used by someone with money to pay. I recall a few years ago someone quoting the widow of the author of Bring Us A Barrel as saying that if she had a pound for each time it was sung, she'd have been a millionaire. Unfortunately in all probability the combined total earned by all those singers was (after expenses) rather less than a pound. Composers in the traditional vein who has earned serious money from their music are rare in these islands.

(2) Nationality and cultural appropriation. Irish music has had a far higher public acceptability (I'm talking aboutthe UK) than English or Scottish music, which may be traced to the "Celtic Twilight" meme of the late Victorian era, Yeats, George Herbert, Lady Gregory and that lot (Sean O'Casey was quite amusing about this, as was Myles na Gopaleen in The Poor Mouth). So whereas English tradition is fatally associated in the stereotype with sweater- clad finger- in- ear Morrismen, and Scottish music with military bagpipes, Irish or Celtic is associated with an advertising cliche of a girl clad in an embroidered gown, long red hair blowing free in the breeze against a hilltop sunset... OK don't get carried away, but you get the point. It does mean that "Irish" versions of any given trad song are more likely to get published, viewed or bought than "dull" east-insular versions. And if they've appropriated Wille McB, Aragon Mill, Dirty Old Town, Shores of Erin (rendering it meaningless in the process), Wild Rover, Roads and Miles to Dundee, Black Velvet Band and the rest, who is going to take any notice of a pedant's whinge of protest?

(3) The somewhat separate issue of assimilation into tradition. Songs like Dark Island and Bring Us a Barrel were deliberately composed to sound like songs already accepted as traditional, and the fact that they have become "anonymous" is in fact a tribute to the skill of their composers. Composers have been deliberately trying to do this since at least the 18th century; see the Ossian controversy, the excellent Earl of Totnes (recorded by the Dransfields), and a whole lot of Burns and Walter Scott. In their case it's a bit like the art forger Tom Keating, whose paintings fooled even experts for a long time (and probably still do). Dark Island (the tune) was being played in Irish sessions around Manchester (UK) at least as early as 1972, when we thought of it as traditional Scottish and none the worse for that. The fact that the Irish tradition took up songs and tunes so much more readily is probably a reflection of the vigour and adaptability of that tradition, and that they were not attributed is more due to the fact that everyone assumed they were traditional than that anyone was trying to suppress authorship.

(4) The inverse case of traditional material being (deliberately or unconsciously) appropriated and copyrighted by modern composers. This seems to be most common in the USA, perhaps because of the bigger market making the stakes higher, and the more aggressive litigation culture. But it certainly happened here too with Sharp and others assuming the copyright of material they collected.

(5) The perceived ethnicity of music, though I don't think it really applies in this case. But the Ottoman Empire of the 1900s, the same recording could be issued labelled as Greek and Hebrew, with only the tune and band names changed to appeal to the different market sectors.

(6) Recomposition and relocation has been a constant process in traditional music. Which fair was Young Rambleaway a-going to? Not knowing the words to Dark Island (as I said, we played the tune), I looked around the web, and found half a dozen variants with different authorial attributions, some of which give a definite location. Which returns us to point 1. What is the copyright of a derivative text when the original is still in copyright?