The song/prayer is actually the last two verses of Pange Lingua, a hymn written by St Thomas Aquinas.

Tantum ergo

Tantum ergo Sacramentum
Veneremur cernui
Et antiquum documentum
Novo cedat ritui:
Praestet fides supplementum
Sensuum defectui.
Genitori, Genitoque
Laus et iubilatio,
Salus, honor, virtus quoque
Sit et benedictio:
Procedenti ab utroque
Compar sit laudatio.
Amen.


V. Panem de caelis praestitisti eis.(T.P. Alleluja)
R. Omne delectamentum in se habentem.(T.P. Alleluja)

Oremus: Deus, qui nobis sub sacramento mirabili, passionis tuae memoriam reliquisti: tribue, quaesumus, ita nos corporis et sanguinis tui sacramysteria venerari, ut redemptionis tuae fructum in nobis iugiter sentiamus. Qui vivis et regnas in saecula saeculorum.

R. Amen.

Following is in the translation in English:

Down In Adoration Falling

Down in adoration falling,
Lo! the sacred Host we hail,
Lo! oe'r ancient forms departing
Newer rites of grace prevail;
Faith for all defects supplying,
Where the feeble senses fail.
To the everlasting Father,
And the Son Who reigns on high
With the Holy Spirit proceeding
Forth from each eternally,
Be salvation, honor blessing,
Might and endless majesty.
Amen.


R. Thou hast given them bread from heaven.
V. Having within it all sweetness.

Let us pray: O God, who in this wonderful Sacrament left us a memorial of Thy Passion: grant, we implore Thee, that we may so venerate the sacred mysteries of Thy Body and Blood, as always to be conscious of the fruit of Thy Redemption. Thou who livest and reignest forever and ever.

R. Amen.