Philadelphia was one of the many northeastern cities that saw heavy Irish immigration in the 19th and early 20th centuries. That automatically makes the Catholic Church records of the area a valuable genealogical resource. The Archdiocese of Philadelphia covers the city and county of Philadelphia, along with Bucks, Chester, Delaware and Montgomery counties. It was founded in 1808 from the Archdiocese of Baltimore and originally encompassed all of Pennsylvania, Delaware and some of New Jersey.
The main place of research is the Philadelphia Archdiocesan Historical Research Center (PAHRC) in Wynnewood, PA. At the center there are many sources of interest to genealogists: parish histories; Catholic newspapers; manuscripts; and periodicals are just some of the resources.
Most importantly, though, are the baptismal and marriage registers, along with records for three orphanages (St. John’s Orphan Asylum, St. Joseph’s House for Homeless Boys, and St. Francis’ Industrial School in Eddington). An important year of reference for research in the registers is 1920. After this year, the records are at the individual parishes and for inquiries before this year you need to contact the center (they don't seem to reference the year 1920 itself, but it is presumed they are at the center too).