Dear njsunsh:
There is a church baptism record for a William Brenan baptised in Castlecomer parish to parents Denis Brenan and Mary Tool on 25 April 1814. Sponsors were listed as Michael Brenan and Margaret Boot.
The digitised image is located in the left top corner of the page here: https://registers.nli.ie/registers/vtls000635329#page/28/mode/1up
As to Mary O'Keefe, there are 3 possibilities in the following parishes: Gowran, Templeorum and St. Patrick's. Interestingly enough, there is an O'Keefe family with two sons born in the 1814 and 1815 years in Castlecomer whose parents are: Patrick O'Keefe and Eleanor Brenan.
It would appear that Castlecomer may be a good place to start unless you have other information.. Did William Brennan and Mary O'Keefe marry in Ireland or in the US?
I see a marriage record for a William Brenan and a Mary Keefe in Thomastown parish dated 10 February 1841. Witnesses to this marriage were: John Reddy and Catherine Pendergast. The record also notes "Knockanore" which is a townland in Thomastown parish. This is most likely where this Mary Keefe resided. If this is the correct couple, then Mary Keefe was most likely a native of Thomastown parish and resided in Knockanore. The tradition here was that brides almost always married in their home parish.
This record can be viewed at the link below: https://registers.nli.ie/registers/vtls000631965#page/158/mode/1up
The digitised images of the above are available on the National Librrary of Ireland website under the parish registers for free. The transcriptions that I noted are availabe to view (fee paying) through Roots Ireland.
Please let me know if you need any further information. I will try to get volunteers from the particular areas to meet with you if possible.
Kind regards,
Jane