Hi Laura,
It sounds like you and I are on parallel tracks--probably not related but going through some of the same search frustration. I've been hunting for info about Mathew Shanly and Mary McGirl (multiple spelling options) who were in Toronto by the late 1840s (and Mathew died there in 1854). In the process I have learned a lot about a whole variety of McGirls, McGurrels, McGuirls, and so on. I've looked for every McGirr, McGuirl, etc. in Ontario and Montreal (including a number of Hughs).
In the process I've gone through many, many Leitrim records with no luck (I'm assuming Leitrim to be the best bet for finding a McGirl and a Shanly in some proximity). However, I've come across the name Magauran often and learned it apparently can also be spelled McGovern. There were a lot of McGirls in Fenagh Parish before the Famine, but apparently Fenagh lost many of its inhabitants in the Famine (says Gerard Macastasney). One of the main McGirl families has an intermarriage between a McGirl and a McGovern (again, spelled a variety of ways). Patrick McGirl married Sarah McGovern ca. 1820. They had 11 or so children and ended up near St. Louis, U.S. (as did one of my McGirls who left Toronto, by the way). I believe one of their sons died in Chicago. So you might check Fenagh and Missouri (there were some other McGirls who landed in Providence, R.I).
(I also wonder whether the townland in Mohill named Drumard Magerraun has some connection to McGurns or Magaurans.)
Meanwhile, I have turned to DNA and to chromosome mapping in my quest. I'm using DNA Painter, and it's interesting, addictive, and in some ways more baffling than not using it. I knew I had ancestors from Waterford County and Ulster Scots from Armagh, but via MyHeritage DNA and DNA Painter, I'm discovering I have a lot of matches with people from Omagh, Co. Tyrone. Were they Ulster Scots? Were they McGirls who had converted from McGurrell to McGirl and moved South? (By the way, I would like to know what sort of DNA testing tells you (or someone) that you have midland ancestors.)
I hope Roger, Jane, or Kevin can help with my issues also (and I'm sure there's a Flynn in my ancestry somewhere). Briefly, my Mary McGirl was the daughter of John McGirl and Elizabeth Misset; she would have been born between 1820 & 1830, I would guess). Misset also has a number of variant spellings, and there were Missets who married Macgaurans or Macgowns and Flynns. The only part of Leitrim where I find all three names is Kiltoghert, but the only part where I find any Shanlys named Mathew is farther south (e.g., Bornacoola).
Susan MacDonald