there is an established tradition of turning traditional music into classical music or parlour music
think Bela Bartok
think Marjorie Kennedy-Fraser (if you are not familiar with her work, Keberoxu, you may wish to read up on her)

sometimes it works the other way around - the replacement of more rustic instruments with fiddles (violins), the way composers such as Carolan have been absorbed into the Irish folk tradition

I think Curraghmór is wrong about the tune predating the lyrics, as there are many Irish language versions of Róisín Dubh. The song is a love song, but Róisín Dubh is also a symbol of Ireland,so I can imagine -as Curraghmór says - situations in which it was acceptable to play the tune alone but that the players and listeners might be aware of a rebeilious undertone.

Brian O Domhnaill says he learned a standard school version of Róisín Dubh before he learned there was a local version in his native Donegal.

'Gus a Róise ná bíodh brón ort fár éirigh duit
Tá na bráithre ag teacht thar sáile is iad triall thar muir
Tiocfaidh do phárdún ón Phápa is ón Róimh anoir
Is ní sparálfar fíon Spáinneach ar mo Róisín Dubh.

'Gus a Róise mhín mhodhmhair 's na gciabhfholt dubh
Tar ag triall chun mo thórraimh más áil leat é
Beidh mo chonair-se tógáilte i lár an lae
Is gurb í do phóg-sa Dé Domhnaigh a bhásaigh mé.

Dá bhfeicfeá-sa Róise Dé Domhnaigh is í ag éirí amach
Crios Phroinsias uirthi timpeall fá lár a coirp
Tháinig an t-am uirthi ar shantaigh sí an t-ógfhear deas
Is mura dtéigh bang uirthi millfidh sí an t-ord ar fad.

'Gus a Róise, dá mba liom thú nárbh aoibhinn duit
Is deas a chealgfainn do leanbhán dá mbeadh sé ag goil
Is in Albain dá gcasfaí sinn, inné nó inniu
Scéal cinnte, a ghrá, go bpillfeadh muid araon gan guth.

Nach aerach a théid sí chun aonaigh ina cóta cuilt
Is gan aici maoin shaolta nó bólacht cnoc
Ná an aon gine déag a bhí faoin a cófra glais
Nach siúd an féirín lenar bhréagnaigh mé mo Róisín Dubh.

Orú mhearaigh tú mé, a bhradaigh, is nár ba fearrde duit
Is go bhfuil m'anam istigh i ngean ort 's chan inné nó inniu
Orú d'fhág tú lag anbhann mé i ngré is i ncruth
Ná feall orm gus mé i ngean ort , a Róisín Dubh.

Is tá réalt ins na spéirthí in imeall an cheo
Is cha raibh á leithéid i nGleann Éidhinigh 'gus ní bheidh go deo
Gaoth na hÉirne go dtugfhas léim leí cé gur mhór an sruth
Is mar gha gréine í ar mhalaidh shléibhe a bhí mo Róisín Dubh.

Beidh an Éirne ina tuille tréana is réabfar na cnoic
'S beidh an fharraige ina tonnaí dearga is doirtfear fuil
Beidh gach gleann sléibhe ar fud Éirinn is na móinte ar crith
Lá éigin sula n-éagfaidh mo Róisín Dubh.