Assuming it is the marriage of Patrick McGivern and Mary ? that you are looking for I
may have solved the mystery (or at least part of it).
It seems as though the records already located are-
1) Rachel's marriage in 1894 listing father as Patrick McGivern, ex-soldier
2) 1901 census with Rachel age 30 born Co.Down and her mother Mary listed as Magertion, age 60, born Co.Down, Church of Ireland, married.
Thinking the spelling of the surname was causing problems finding a marriage record, I searched for McGivern, McGovern, Magowan, McG*, etc. Not finding anything even close (anywhere in Ireland) I then decided to search for Rachel's birth (c1871 Co.Down with parents Patrick & Mary)- again nothing. So, removed Patrick's name from the search and...
Rachel Mcgiverin born 23 Sepr.1866 Co.Down- mother Mary Mcgiverin
https://familysearch.org/pal:/MM9.1.1/FPSC-SY7If I'm correct, Rachel was illegitimate, born c few years before 1901 census age and made up a father's name and occupation when she got married. Since Mary's listed in 1901 as 'married' it could be that she kept up the story of Rachel's father and Rachel only gave the details as she was told them.