I know wikipeadia is often mocked as a resource, but sometimes they have solid articles. I think their article on St. Philomena is pretty good. Excerpt:
Saint Philomena was, as believed by her devotees within the Catholic Church, a*** young virgin martyr*** whose remains were discovered in 1802 in the Catacombs of Priscilla. Three tiles enclosing the tomb bore an inscription that was taken to indicate that her name (in the Latin of the inscription) was Filumena, the English form of which is Philomena.
The remains were removed to Mugnano del Cardinale in 1805 and became the focus of widespread devotion, with several miracles credited to the saint’s intercession, including the healing of Venerable Pauline Jaricot in 1835, which received wide publicity. Saint John Vianney attributed to her intercession the extraordinary cures that others attributed to himself.
In 1833 a Neapolitan nun reported that in a vision Saint Philomena had revealed that she was a*** Greek princess martyred at 13 years of age by Diocletian, who was Roman Emperor from 284 to 305.***
The shrine of her relics in Mugnano del Cardinale continues to be visited by pilgrimages from many countries, an Archconfraternity in her honour exists, as does popular devotion in various places around the world.
Discovery of the remains
On 24 May 1802 in the Catacombs of Priscilla on the Via Salaria Nova an inscribed loculus (space hollowed out of the rock) was found, and on the following day it was carefully examined and opened. The loculus was closed with three terra cotta tiles, on which was the following inscription: lumena paxte cumfi. It was and is generally accepted that the tiles were in a wrong order and that the inscription originally read, ***with the leftmost tile placed on the right: pax tecum Filumena (i.e.“Peace with you, Philomena”***). Within the loculus was found the skeleton of a female between thirteen and fifteen years old. Embedded in the cement was a small glass phial with vestiges of what was taken to be blood. In accordance with the assumptions of the time, the remains were taken to be those of a virgin martyr named Philomena.
In 1805, Canon Francesco De Lucia requested relics for a new altar, and on 8 June obtained the remains discovered in May 1802 (reduced to dust and fragments) for his church in Mugnano del Cardinale, where they arrived on 11 August, after being taken from Rome to Naples on 1 July.
In 1827, Pope Leo XII gave to the church in Mugnano del Cardinale the three inscribed terra cotta slabs that had been taken from the tomb.
Spread of devotion
In his Relazione istorica della traslazione del sagro corpo di s. Filomena da Roma a Mugnano del Cardinale, written in 1833, Canon De Lucia recounted that wonders accompanied the arrival of the relics in his church, among them a statue that sweated some liquid continuously for three days.
A miracle accepted as proved in the same year was the multiplication of the bone dust of the saint, which provided for hundreds of reliquaries without the original amount experiencing any decrease in quantity.
Reported life of the Saint
On 21 December 1833, the Holy Office declared that there was nothing contrary to the Catholic faith in the revelations that Sister Maria Luisa di Gesù (1799–1875), a Dominican tertiary from Naples, claimed to have received from the Saint herself.
…In these visions, Saint Philomena also revealed that … ***her name “Filumena” meant “daughter of light”***. (It is usually taken to be derived from a Greek word meaning “beloved”.)
The spread of devotion to her in France as well as in Italy was helped when ***Saint John Vianney built a shrine in her honour and referred to her often, attributing to her the miracles that others attributed to himself.***
Many other Saints were devoted to St. Philomena, e.g., ***St. Peter Julian Eymard***, St. Peter Chanel, St. Anthony Mary Claret, St. Madelaine Sophie Barat, St. Euphrasier Pelletier, ***St. John Neumann***, and Bl. Anna Maria Taigi.
In his book It Is Time to Meet St Philomena, Mark Miravalle says that Pope Gregory XVI “liturgically canonized Philomena, in an act of the ordinary Papal Magisterium”. This contrasts with the usual view that canonization is an exercise of infallible magisterium declaring a truth that must be “definitively held”.
The Roman Martyrology contains the names of all the saints who have been formally canonized, since “with the canonization of a new saint, that person is officially listed in the catalogue of saints, or Martyrology”, and “as soon as the beatification or canonization event takes place, the person’s name is technically part of the Roman Martyrology”. It does not now contain and in fact never included the name of this Philomena, which can be seen to be absent in the 1856 edition published some twenty years after the 1837 decree.
In 1961 two Catholic periodicals, America and Commonweal, published articles asserting that St. Philomena was “never canonized.”  
Of course, lack of canonization does not mean lack of sainthood. Canonization was introduced only after many centuries of the Church’s existence, and for that reason none of the saints mentioned in the Roman Rite Canon of the Mass were ever canonized.