(In this post I use the first numbering in the Complete Peerage, i.e. starting from a creation in 1228 to Miles de Courcy (d. c. 1230) and ignoring a doubtful 5th Baron. Lots of different numberings can be encountered in different sources - I have tried to include relevant dates to help those looking at other sources)
I've had a look at this - it is clear from a cursory check of the Complete Peerage that the title has only ever descended to males, with no abeyances or female Baronesses. This is corroborated by the reference in the Complete Peerage under the 24th Baron (d. 1776) to this being an "ancient Baron[y] which [has] always descended in the male line". This was decided in 1761 by the Irish House of Lords. It is clear that the Nicholas de Courcy, the 11th Baron (d. 1475) was the heir general
and heir male of the first Baron. His son, James de Courcy, the 12th Baron (d. 1491 or 1499) succeeded normally, but it then gets a bit murky, as the Edmund, the 13th Baron and surviving son was an idiot and was displaced by his uncle, David, the 14th Baron (d. 1520). It is unclear when Edmund died, but in any event he d.s.p. (childless). If the title was capable of descending on female lines, it would never have gone to David, and instead to Edmund's niece, Anastasia de Courcy. Anastasia did marry, and her descendants would be the heirs general, if the title were capable of descending in that fashion. It would appear that the decision in 1761 simply confirmed the descent as it had been occurring to that date, i.e. heirs male of the body.
Wikipedia suggests that the title was created by writ, and that a later licence was interpreted as a patent by some. The Complete Peerage itself states that the origins are "obscure".
In short, to address Anna's query:
- No, the descendants of H. Emily Letitia de Courcy are not in line to the title as this is not a male line as established at the latest by the Irish House of Lords in 1761 (and probably earlier).
- If you could somehow reverse the ruling and allow this title to descend along female lines (this would be an eye-watering expense in litigation with little chance of success in my view), then the title would not seek out female heirs of relatively recent Barons, but instead would descend to the heir general of Anastasia de Courcy above. It may be that it would not be possible to definitively trace those heirs and the title would fall into dormancy.
(Please also try to keep your replies on the same thread so that it is easier to follow, rather than creating new threads)