From Dom Guéranger's The Liturgical Year.

TO-DAY a precursor of Laurence appears on the cycle, the deacon Cyriacus, whose power over the demon made hell tremble, and entitles him to a place among the saints called helpers. He and his companions in martyrdom form one of the noblest groups of Christ's army in that last and decisive battle, wherein the eagerness of the faithful to show that they knew how to die won victory for the Cross. Rome, baptized in the blood she had shed, found herself Christian in spite of herself; all her honours were now to be lavished upon the very men whom in the time of her folly she had put to the sword. Such are Thy triumphs, O Wisdom of God!

Mention of the three martyrs celebrated to-day is to be found in the most authentic calendars of the Church that have come down to us from the fourth century.[1] If then, as Baronius acknowledges,[2] there is some reason for calling in question certain details of the legend, their cultus is none the less immemorial upon earth; and the unwavering devotion of which they are the objects, especially in the sanctuaries enriched with their holy relics, proves that they have great power before the throne of the Lamb.

Cyriacus diaconus, cum Sisinio, Largo, et Smaragdo diutius inclusus in carcere, multa edidit miracula, in quibus Arthemiam Diocletiani filiam precibus a dæmone liberavit: missusque ad Saporem Persarum regem, Jobiam etiam ejus filiam a nefario spiritu eripuit. Rege vero ejus patre cum quadringentis ac triginta aliis baptizatis Romam rediit: ubi Maximiani imperatoris jussu comprehensus, catenis vinctus ante rhedam suam trahitur: et post dies quatuor e carcere eductus, pice liquata perfusus, et in catasta extensus, demum cum Largo et Smaragdo, aliisque viginti securi percussus est via Salaria, ad hortos Sallustianos. Quorum corpora in eadem via, decimo septimo Kalendas Aprilis, sepulta a Joanne presbytero, postea sexto idus Augusti a Marcello pontifìce, et Lucina nobili femina lineis velis involuta, et pretiosis unguentis condita, in ipsius Lucinæ prædium via Ostiensi, septimo ab urbe lapide translata sunt.
Cyriacus, a deacon, underwent a long imprisonment together with Largus, Sisinius and Smaragdus, and worked many miracles. Amongst others, by his prayers, he freed Arthemia, a daughter of Diocletian, from the possession of the devil. He was sent to Sapor. King of Persia, and delivered his daughter, Jobia, in like manner from the devil. He baptized the king, her father, and four hundred and thirty others, and then returned to Rome. There he was seized by command of the Emperor Maximian, and dragged in chains before his chariot. Four days afterwards he was taken out of prison, boiling pitch was poured over him, he was stretched on the rack, and at length he was put to death by the axe, with Largus, Smaragdus, and twenty others at Sallust’s Gardens on the SalarianWay. A priest named John buried their bodies on that same way, on the seventeenth of the Kalends of April, but on the sixth of the Ides of August Pope Marcellus and the noble lady Lucina wrapt them in linen with precious spices, and translated them to Lucina’s estate on the Ostian Way, seven miles from Rome.

The Church to-day recites this prayer in their honour:

Prayer

Deus, qui nos annua sanctorum martyrum tuorum Cyriaci, Largi et Smaragdi solemnitate lætificas: concede propitius; ut quorum natalitia colimus, virtutem quoque passionis imitemur. Per Dominum.
O God, who dost rejoice us by the annual solemnity of Thy holy martyrs, Cyriacus, Largus and Smaragdus, mercifully grant that we may imitate the virtue with which they suffered, whose festival we celebrate. Through, etc.

[1] Calendarium Bucherii.
[2] Annal, ad An. 309, vi.