Greetings from the U.S. I'm sure folks in Cavan will be able to help in more detail, but a couple of basics: Crosserlough is a Parish, and Drummlonan is a Townland within the Parish of Crosserlough. If you look at the Griffiths Valuation of the 1840s, there are 48 McGivneys listed: 28 live in Crosserlough, 10 live in the Parish of Denn (which borders Crosserlough), and 4 live in the Parish of Lavey (which borders Denn). So the McGivneys stuck together, and that should give you hope in tracing your Mary Ann.
The U.S. angle to all this is that a McGivney was one of the most significant figures in the nineteenth century American Catholic Church: Fr. Michael McGivney founded the Knights of Columbus, and the cause for his canonization was opened a few years ago in Rome. (For what it's worth, he seems genuinely to have been a holy man.) His father Patrick McGivney (b. 1825) emigrated from Cavan in 1849, and lived in Waterbury, Connecticut where he married Mary Lynch (likewise from Cavan). They had 12 children, seven of whom survived to adulthood. The first official biography of Fr. McGivney was published in 2006 ("Parish Priest: Father Michael McGivney and American Catholicism"), and steers clear of naming the Cavan parishes his parents came from. So I do not know if there is lingering uncertainty there. Fr. McGivney's youngest brother, John, became a priest himself, and ended up as the Parish priest to my family, baptizing my father and uncles.
So we have reason to be grateful to the McGivney clan! Again, best of luck with your Mary Ann.
