Michael O'Callaghan family

[Nana41]

Hello, today it was suggested i try this site.  I am searching for the birth certificate of my Paternal grandmother Margaret O'Callaghan Langdon who was born 17 December 1877 Midleton Cork to Michael  O'Callaghan who is named on her 1901 marriage certificate in England. Michael's occupation is a Fitter.  I have no idea what that occupation is. Margaret's middle name may be Josephine.  Margaret married Frederick William Hartnup Langdon, son of Robert Rockett Langdon, oldest Master Mariner of Bristol England. 
Margaret had a sister Mary O'Callaghan Webber. I have sent to the GRO in England for many of my family but have no idea how or where to search and order certificates in Ireland.  Any help is sincerely appreciated. 
Thank you, 
Nancy Langdon Webb,  United States Of America.     

m

[Sinann]

Certs don’t need to be ordered they are free on Irish Genealogy https://www.irishgenealogy.ie/en/
You just need to sign in and prove you’re not a robot.
Couple of points
A Fitter is a person who installs or puts together machines of any kind.
Did the entire family move to England, if so Michael may not have been a fitter in Ireland when Margaret was born.

The O of O’Callaghan may not have been used in the 1870s, it’s commonplace for O Mc etc to be missing for generations and added on in the early 1900s.

You have an exact date, but I don’t see a birth certificate for Margaret with that date and this is often the case, it’s not unusual for people to be months or even years out with their birthday.

I do see as a possible a Margaret Callaghan 16 Dec 1874 to Michael Callaghan and Ellen Murphy

https://civilrecords.irishgenealogy.ie/churchrecords/images/birth_returns/births_1874/03122/2145207.pdf
This girl was RC
her baptism 20 Dec, first names are in Latin.

https://registers.nli.ie/registers/vtls000634732#page/59/mode/1up

These records are Midleton registration district in Cork and Middleton Parish in Cork, as per your post but it’s posted on the Mayo board you can use the report to moderator to have it moved to the Cork board.

[Kiltaglassan]

Welcome to RootsChat 

Further to Sinann's finds, elder sister Mary born 3 November 1872. Father Michael a millwright.
https://civilrecords.irishgenealogy.ie/churchrecords/images/birth_returns/births_1872/03220/2180511.pdf

Michael's marriage to Ellen Murphy - 26 November 1871
https://civilrecords.irishgenealogy.ie/churchrecords/images/marriage_returns/marriages_1871/11365/8172629.pdf

Millwright
https://en.wikipedia.org/wiki/Millwright