From the martyrology:
"The holy martyrs Cyriacus, deacon, Largus, and Smaragdus, with twenty others who suffered on the 16th of March, during the persecution of Diocletian and Maximian. Their bodies were buried on the Salarian Way by the priest John, but were on this day translated by Pope St. Marcellus to the estate of Lucina, on the Ostian Way. Afterwards they were brought to the city and placed in the church of St. Mary in Via Lata."St Cyriacus was a Roman nobleman who converted to Christianity as an adult and, renouncing his material wealth, gave it away to the poor. He spent the rest of his life ministering to the slaves who worked in the Baths of Diocletian.
Under the reign of Western Roman Emperor Maximian, co-emperor with Diocletian, St Cyriacus was tortured and put to death, beheaded in 303 on the Via Salaria, where he was subsequently buried. With him were martyred his companions Largus and Smaragdus, and twenty others, including Crescentianus, Sergius, Secundus, Alban, Victorianus, Faustinus, Felix, Sylvanus, and four women: Memmia, Juliana, Cyriacides, and Donata.
Saint Cyriacus is credited with exorcizing demons from two girls. The first was Artemisia (or Artemia), the daughter of Emperor Diocletian, which resulted in both Artemisia and her mother Saint Serena converting to Christianity. The second was Jobias, the daughter of Shapur II of Persia (reigned 241-272), which led to the conversion of the King's entire household.
He is one of the fourteen holy helpers.