Seeking parentage of Elizabeth Sheridan

Hello! I am in dire need of help finding my great grandmother's parentage. I have census records from 1900 straight through to 1940 all indicating father and mother are from Ireland. Death record lists father as Phillip Sheridan but nothing for the mother. I have tried family search, NYC Archives and ancestry all to no avail. My ancestor is Elizabeth Sheridan sometimes she pops up as Lizzie born 1868.  I found an index for her marriage certificate but no image was available for further information. I am currently waiting for a response from the Mt Vernon clerks office. I have checked asylum ,orphanage, prison and even bodies in transit archives. Can someone please help me with this? I can't find a birth certificate or baptism record for Elizabeth Sheridan  born July 1, 1868 this is confirmed by her death record. Thank you in advance for your help.

Here is all the information that I have: Elizabeth "Lizzie" Sheridan; Born July 1, 1868 in New York; Died April 10, 1948 in New York City; Married Matthew Kiernan around 1892. Could possibly be September 12, 1893 in Mt Vernon NY; Siblings: Unknown; Parents: Per death certificate Philip Sheridan born in Ireland was listed. No Mother's name listed.  From the census records that I can find 1900 through 1940 Lizzie has lived in NYC (Manhattan). Also, every census has Lizzie's Parents as both being born in Ireland.  My information has been cross referenced with numerous sources. Thank you for your help.