After doing some research it appears that Nancy seems to be referring to her husband's death in the Wexford Rebellion that occurred in County Wexford, Ireland in the year 1798 (more of which can be read about here: https://en.wikipedia.org/wiki/Wexford_Rebellion). Her husband, Michael Kelly (aged 59 at the time of his death) was likely part of the rebel group that was sent west towards New Ross during the second phase of the Wexford Rebellion, where they engaged British troops outside and later within the town in a vicious battle which ultimately ended up resulting in a British victory. Michael was likely one of the estimated 2 800 - 3 000 casualties that had occurred on the rebel side during or after the fighting (for more info on the battle in general, feel free to look here: https://en.wikipedia.org/wiki/Battle_of_New_Ross_(1798)).
There does seem to be a profile for Nancy (albeit with Merryman spelled Merriman) matching most of the details provided, yet, according to the profile, it seems that her maiden name was actually Nancy Merriman/Merryman rather than Anne Keefe. In addition to that, I haven't been able to find an Anne Keefe which matches the details provided and the information that I've gathered so far.
To add on to what you said about when she wrote the letter, it seems that she wrote it to Commodore John Barry on the 20th of May, according to documents I found online.
For Nancy herself, she appears to be a cousin of Commodore John Barry, according to irishamerica.com, and based on what I've read in documents regarding John Barry online, it seems that Nancy was the eldest daughter of Margaret, John Barry's sister, making Nancy quite a close relative of Mr. Barry.
In a fair portion of the documents I found online, the spelling of Nancy's surname matches the one provided in the post you made above. Chances are Merriman could have been an alternative spelling of Merryman, or perhaps vice-versa.
I'd recommend checking the internet archives (archive.org) if you're interested in doing some further research, as there are quite a few scanned documents which contain both spelling variants of the Merryman/Merriman family name. Interestingly, a lot of USPTO Patents Applications seem to contain the surname Merriman within its pages.
Nancy (Merriman) Kelly's Profile on WikiTree:
Hope this helps :)
Also, if anyone notices any errors or inaccuracies within my answer, feel free to let me know and I will happily correct them.