Quite simply, Roman Catholics have interpreted the text of 1 Peter 3:19 to mean that Jesus Christ descended into Purgatory for admonition purposes. In other words, a number of Catholic apologists have identified the "spirits in prison" to be professing Christians suffering in the purifying flames of Purgatory.
This text is not referring to human beings suffering in Purgatory, but rather concerns Christ descending into Hades for the purpose of proclaiming to the fallen angels. It means that the same Holy Spirit of God who resurrected Jesus Christ from the grave also enabled Him to use Noah as an instrument to preach repentance to other men during his earthly lifespan (during the construction of the ark which took place prior to the Genesis flood). Jesus preached the message of His triumph over sin and death to the fallen angels who have been imprisoned since the time of the flood. 1 Peter 3:19 is referring to a place for nonbelievers. This footnote from the Roman Catholic New American Bible Revised Edition should settle the matter:
"3, 19: The spirits in prison: It is not clear just who these spirits are. They may be the spirits of the sinners who died in the flood, or angelic powers, hostile to God, who have been overcome by Christ (ch 22; Gn 6, 4; Enoch 6-36, especially 21, 6; 2 Enoch 7, 1-5)."