Yes Hallmark I agree it fits for both but there are more names with this reference
Dublin South; Year 1868; Quarter 1; Volume 7; Page 685
When you look at all four names and other records
James MAHON m Anne KEOGH and Patrick DELANY m Mary FitzSIMONS
editted to add quote from original post
I have a possible marriage for them in 1868 and she may go by the name of Mary Fitzsimons?
The name FitzSIMONS seems to only come from this record which is not a match to a James MAHON at all
The combination of James MAHON and Mary FitzWILLIAM(S) is documented from at least 1864 up to 1897Also James MAHON and Mary FitzWILLIAMS had at least six children while living in Tallaght
I was looking at the familysearch.org link given by dathai and civil records
1864 Jun 17 James
1866 Jun 25 John
1868 Jul 03 Maria/Mary
1871 Feb 05 Sarah
1873 Jun 03 Patrick
1876 Jan 15 Eliza
all show mother as Mary FitzWILLIAMS
also Eliza MAHON married Charles POWER on Feb 28 1897 in Rathmines where her parents are still shown as James MAHON and Mary FitzWILLIAM now of Kimmage
LINK #685
Other MAHON marriages from the 1889 to 1897
LINKwith a possibility that twins Peter and Paul MAHON b Sep 11 1877
LINK #384 & 385 in Kimmage
were also their sons; mother shown as WILLIAMS
Baptism of Mary Anne MAHON Oct 05 1884 in Rathmines
LINK