You may already be aware of them, but there are records for that Hornsby-McDowell family in the FamilySearch database [https://www.familysearch.org]. If you search there, you can find Martha under ID number 9JVM-DCC, and can access the records for the others easily through that record, as you will see.
FamilySearch is operated by the LDS church, and is free to use. You have to create an account to use the database, but they don’t ask for much personal info, and no missionaries will contact you or anything like that. The database does not have separate family trees, and is essentially one huge tree of linked records, in which there is a single record for each person, with a unique identification number (although sometimes people unknowingly create duplicate records for the same person, which you can merge when you spot them).
The entries in Martha’s record do not add much to what you already have, except that they show Martha and her husband, Joseph, as having seven children, and that Sarah was not the first-born. Instead, a son named John is shown as the eldest. His record shows that he lived until 1909, and had nine children of his own. In each record, at the right hand side of the screen, you can see the screen names of those who have created or modified the record, and can send them private messages, so you might be able to get more info by contacting them (and perhaps also locate some new relatives that way).
The MacDowell surname [in its Gaelic form, Mac Dubhghaill, meaning "son of the black foreigner", sometimes also anglicized as MacDougall] is common in Ulster, particularly among those who migrated from Scotland since the 17th century, but there were also Mac Dubhghaill’s who migrated to County Roscommon from the Hebrides centuries earlier, before the Reformation, as what were called “gallowglasses” (foreign mercenaries, or gallóglaigh in Irish). Gallowglasses were often recruited from Scotland to assist local Irish or Norman-Irish lords in their disputes with one another, and they frequently settled down and became part of the local population (many people in Ireland have gallowglass ancestors). Most remained Catholic after the Reformation, like the general Irish population. Athlone borders on County Roscommon, so there were probably at least some MacDowell’s in the area. If Joseph met Martha there, then you might be able to search the (Catholic) Athlone parish register for a baptismal record for Martha, which should also give you the names of her parents.
The Athlone parish registers which are available online go back to 1813 (with some gaps), and the ones for St. Mary's parish in Athlone can be accessed at this link:
https://registers.nli.ie/parishes/0178
Luckily, the target year you have (1843) doesn’t fall in one of the gaps, but be prepared to treat that year as a starting point, because people in those days often did not really know when they were born. The registers appear to cover several years before and after that year, though, so you might be lucky and find her.
There is a second Athlone parish you can search, St. Peter’s, which is actually in Roscommon, just across the county boundary. The records for that parish go back to 1789, and can be accessed here:
https://registers.nli.ie/parishes/0538
mso-fareast-font-family:Calibri;mso-fareast-theme-font:minor-latin;mso-bidi-theme-font:
minor-bidi;mso-font-kerning:0pt;mso-ligatures:none;mso-ansi-language:EN-US;
mso-fareast-language:EN-US;mso-bidi-language:AR-SA">Unfortunately, that set of registers also appears to have gaps, and 1843 appears to fall in one of them. Even so, you might be able to locate some of Martha’s siblings, since you have her father’s name, if there are not too many MacDowell’s in the register.
