From the Cambridge Lessons and Carols Yesterday

After the Annunciation reading from the Gospel according to St. Luke, the Choir of King's College Cambridge sang this happy hymn:

1. Angelus ad virginem
Subintrans in conclave.
Virginis formidinum
Demulcens inquit "Ave."
Ave regina virginum,
Coeliteraeque dominum
Concipies
Et paries
Intacta,
Salutem hominum.
Tu porta coeli facta
Medella criminum.

2. Quomodo conciperem,
quae virum non cognovi?
Qualiter infringerem,
quae firma mente vovi?
'Spiritus sancti gratia
Perficiet haec omnia;
Ne timaes,
sed gaudeas,
secura,
quod castimonia
Manebit in te pura
Dei potentia.'

3. Ad haec virgo nobilis
Respondens inquit ei;
Ancilla sum humilis
Omnipotentis Dei.
Tibi coelesti nuntio,
Tanta secreti conscio,
Consentiens
Et cupiens
Videre
factum quod audio,
Parata sum parere
Dei consilio.

4. Angelus disparuit
Etstatim puellaris
Uterus intumuit
Vi partus salutaris.
Qui, circumdatus utero
Novem mensium numero,
Hinc Exiit
Et iniit
Conflictum,
Affigens humero
Crucem, qua dedit ictum
Hosti mortifero.

5. Eia Mater Domini,
Quae pacem reddidisti
Angelis et homini,
Cum Christum genuisti;
Tuem exora filium
Ut se nobis propitium
Exhibeat,
Et deleat
Peccata;
Praestans auxilium
Vita frui beta
Post hoc exsilium.

This site gives some background:

The cheerfully sounding song about the Annunciation, Angelus ad Virginem or, in its English form, Gabriel, From Heven King Was To The Maide Sende, was a popular Medieval carol that is still popular today. The text of this song is a poetic version of Hail Mary, full of dramatic tension and theological profundity.

It appeared in an Dublin Troper (c. 1361, a music book for use at Mass) and was found in a Sequentiale (Vellum manuscript, 13th or 14th century), possibly connected with the Church of Addle, Yorks. This lyric also appears in the works of John Audelay, in a group of four Marian poems. Audelay may have been a priest; he spent the last years of his life at Haghmond, an Augustinian abbey, and wrote for the monks there.

It is said to have originally consisted of 27 stanzas, with each following stanza beginning with the consecutive letter of the alphabet.

Chaucer mentions it in his Miller's Tale, where poor scholar Nicholas sang it in Latin to the accompaniment of his psaltery:

And over all there lay a psaltery
Whereon he made an evening's melody,
Playing so sweetly that the chamber rang;
And Angelus ad virginem he sang;
And after that he warbled the King's Note:
Often in good voice was his merry throat.

Both the Oxford Book of Carols and, especially, the New Oxford Book of Carols contain musical settings and additional historical notes.

In addition to the translations provided, there is the translation by John Macleod Campbell Crum, 1932, which is reproduced as #547 in Hymn Ancient & Modern, Revised.

The site also notes:

The carol was probably Franciscan in original and brought to Britain by French friars in the 13th century. There is a 14th Irish source for the latin version and, from the same period, a middle-English version which begins:

Gabriel fram Heven-King / Sent to the Maide sweete,
Broute hir blisful tiding / And fair he gan hir greete:
'Heil be thu, ful of grace aright! / For Godes Son, this Heven Light,
For mannes love / Will man bicome /And take / Fles of thee,
Maide bright, / Manken free for to make / Of sen and devles might.'