These books were all printed before 1923 and therefore should be in the public domain in the US; other countries laws may be different. All of them are available in full text in Google Books and/or archive.org, again in the US at least; copyright laws may prevent the versions of these sites in other countries from including some of these books.
All of these books include at least some Breton traditional songs in musical notation, usually with lyrics in Breton, French, English, or some combination thereof.
There also exist many older books giving the lyrics to Breton folk songs in Breton, French, English, or other languages, but I list here only ones including at least some musical scores.
Folk Songs of Many Lands, J. Curwen, 1911 - Incudes 2 Breton songs on pages 7 and 21.
Trente mélodies populaires de Basse-Bretagne, Henry Lemoine, 1885
Barzaz-Breiz: chants populaires de la Bretagne, Théodore Hersart de La Villemarqué Didier & Cie, 1867 - One of the most important sources.
Ballads and songs of Brittany, by T. Taylor, tr. from the 'Barsaz [sic] Breiz' of vicomte Hersart de la Villemarqué - Based on the above but not a straight translation of it, with some of the songs in scores arranged for voice and piano.
Sixty Folksongs of France, Julien Tiersot Oliver Ditson Company, 1915 - Includes Breton songs on pages 40 and 108.
Forty-four French Folk-songs and Variants from Canada, Normandy, and Brittany Julien Tiersot G. Schirmer, 1910 - Includes some Breton songs.
Chants et chansons populaires de la France: Noëls. Chansons de mai. Ballades. Chansons de métiers. Rondes. Chansons de mariées Dumersan (Théophile Marion, M.), Hippolyte Raymond Colet, Champfleury, Jean-Baptiste Weckerlin, Le Roux de Lincy, Ourry (E. T. Maurice, M.) Garnier frères, 1860 - Has a section on Bretagne, with a few songs with French lyrics, scored for voice and piano.
Breton Folk-songs: Set to Music (for Soprano, Contralto, Tenor and Bass,) Liza Lehmann, Frances Marion Gostling, Chappell, 1909 - Lyrics and notes in English. The translations are of doubtful authenticity and the musical arrangements rather florid, but still a useful source.
3 plus ones
Taylor Ballads and songs of Brittany, by T. Taylor, tr. from the 'Barsaz [sic] Breiz' of vicomte Hersart de la Villemarqué
Curwen Folk Songs of Many Lands J. Curwen, 1911
Villemarqué Ballads and Songs of Brittany Théodore Hersart vicomte de La Villemarqué, tr. Tom Taylor Macmillan and Company, 1865
Lemoine Trente mélodies populaires de Basse-Bretagne Henry Lemoine, 1885
Barzaz-Breiz: chants populaires de la Bretagne Théodore Hersart de La Villemarqué Didier & Cie, 1867
Sixty Folksongs of France Julien Tiersot Oliver Ditson Company, 1915
Forty-four French Folk-songs and Variants from Canada, Normandy, and Brittany Julien Tiersot G. Schirmer, 1910
Ballads and Songs of Brittany Théodore Hersart vicomte de La Villemarqué, Tom Taylor Macmillan and Company, 1865
Chants et chansons populaires de la France: Noëls. Chansons de mai. Ballades. Chansons de métiers. Rondes. Chansons de mariées Dumersan (Théophile Marion, M.), Hippolyte Raymond Colet, Champfleury, Jean-Baptiste Weckerlin, Le Roux de Lincy, Ourry (E. T. Maurice, M.) Garnier frères, 1860
Ancient Poems, Ballads, and Songs: Of the Peasantry of England, Taken Down from Oral Recitation, and Transcribed from Private Manuscripts, Rare Broadsides and Scarce Publications, Volume 17 James Henry Dixon
Percy Society, 1846
English County Songs: Words and Music Lucy Etheldred Broadwood, John Alexander Fuller-Maitland Leadenhall Press, 1893
English County Songs: Words and Music Lucy Etheldred Broadwood, John Alexander Fuller-Maitland Leadenhall Press, 1893
Songs of the Four Nations: A Collection of Old Songs of the People of England, Scotland, Ireland, and Wales, for the Most Part Never Before Published with Complete Words and Accompaniments . B. Cramer, 1893
Journal of the Folk-Song Society, Volume 2, Issues 6-9 Folk-Song Society (Great Britain) The Society, 1906
The Poets and Poetry of Munster: A Selection of Irish Songs by the Poets of the Last Century John O'Daly, 1850
Sixty Irish Songs William Arms Fisher O. Ditson Company, 1915
Sweet Olde Irish Songs: A Selection of Famous Celtic Airs and Ballads Harry C. Phibbs, Robert J. Cole Log Cabin Press, 1914
Irish Country Songs, Volume 1 Herbert Hughes Boosey, 1909
Irish songs: a collection of airs old and new Nathaniel Clifford Page, Oliver Diston Company, 1907
Irish Songs and Poems William Allingham Longmans, Green, 1901
Songs of Erin: A Collection of Fifty Irish Folk Songs Sir Charles Villiers Stanford, Alfred Perceval Graves Boosey & Company, 1901
The Irish Melodies: Op.60 Boosey, 1895
Irish Minstrels and Musicians: With Numerous Dissertations on Related Subjects Regan Printing House, 1913
A General Collection of the Ancient Irish Music,: Containing a Variety of Admired Airs, Never Before Published, and Also the Compositions of Conolan & Carolan; Collected from the Harpers &c. in the Different Provinces of Ireland, and Adapted for the Piano-Forte, : with a Prefatory Introduction by Edward Bunting, Preston & Son, 1796
The Poets and Poetry of Munster: A Selection of Irish Songs by the Poets of the Last Century John O'Daly, 1850
The Isle of Man Agnes Herbert, J. Lane, 1909
Manx national music: selected from the ms. collection of the Deemster Gill, Dr. J. Clague, and W.H. Gill W. H. Gill Boosey, 1898
Manx ballads & music Arthur William Moore, G. & R. Johnson, 1896