I'm not sure that I can be of great help here, but I do have some potentially useful info. My great-grandfather, James Cullen, was born in 1836 in the parish of Daingean in County Offaly, in the townland of Eskermore, so he would be of an age to be a sibling to your Margaret. In fact, he did have a sister named Margaret, but she was born in 1845, so was too young to be your Margaret (also, she married in Daingean in 1872). I've found baptismal records for a number of cousins of James Cullen, and also other relatives tracing through his father (my great-great-grandfather), John Cullen, who was born in Daingean in 1796, but I don't see a suitable Margaret among them. I was searching for the descendants and ancestors of certain people, though (especially people lving in Eskermore), and I may not have noted down the info for Cullen's whose connection I could not establish.
In case you want to check out the records I've found, you can find that John Cullen in the data which I've placed on the Family Search web site [https://www.familysearch.org], with John's ID number there being LVGB-CMX, and trace the family up and down through him (it's easy to maneuver around on that site). I'm not a Mormon myself (it's their site), but it's open to all and I use it because it's the best one I've found, and has the largest genealogical datatbase in the world. It's also free to use!
As you'll see, the family earlier had some sort of connection with the town of Birr, which is on the other side of Offaly. I haven't been able to figure out the history of that, but there are still Cullen's in Birr today (I've tried to contact them, but without success thus far). I did not do extensive searching of records in Birr, but i recently noticed that there are partial census records from Birr for 1821, which you can access at the Irish National Archives site [http://www.genealogy.nationalarchives.ie]. ; There were three Cullen families listed (or actually five, since in two cases two related families seem to be living together), and two of the mothers were named Margaret. From the info you gave, your Margaret would not yet have been born, but it indicates that there may be extensive records of Cullen's in the Birr parish register.
So, you might start your searching in Birr and Daingean. The Cullen surname is not extremely common in Offaly, from what I can tell. I've searched all of the available online parish registers in Offaly for the 1835-1845 time range, although I was mostly looking for another great-grandparent at the time, so I may have overlooked some Cullen's. The Ó Cuilinn and Mac Cuilinn surnames were (and are, as Cullen or variants such as Quillan) much more common in the southeast of Ireland, but from what I've read "our" Cullen "clan" in central Ireland is of a separate origin.
Hope some of that turns out to be helpful, and if we're distantly related, then howdy cousin! I've done DNA testing, by the way, and if you have also and have your results uploaded at GEDMATCH (another site which is free to use: https://www.gedmatch.com/login1.php), you could compare our kits. My kit there is T780556.
Good luck in your search!
-- Kevin Jewell
