Michele,
In the 1901 Irish census there were 13,000 people named Fitzgerald and 1600 of those were named Mary/Maria. The name would have been equally common in the 1840s. You can use this site to see where the surname was most common around 1850. Sadly it’s almost everywhere in Ireland save for the counties of Ulster.
https://www.johngrenham.com/surnames/
I think what you have to do is to try and find Maria’s parents names from her marriage or death certificate, or some other source in the US, to try and track down the right family. Perhaps naturalisation papers might give some additional information? Without parent’s names it’s pretty much a needle in a haystack.
I would also be careful about relying too heavily on Maria’s birth definitely being in March 1842. Most Irish people in the 1800s didn’t celebrate birthdays and rarely knew when they were born. If officialdom later required a birth date, they often just guessed or made one up. Normal advice, in Ireland anyway, is to search 5 years either side of the year given.