Here's my Translations of the Chapter titles. I'm pretty sure many of these have old-Irish style séimhiú (a dot above a consonant denotes what would now be a h after the consonant) in the originals that have not been translated by the OCR, so there are several missing h letters. If I weren't on a plane over Afghanistan, I'd download the PDF to check. Will update the repo when I can!
Pláinéid na feaca Súil Duine riamh = A planet no person's eyes have ever seen
An Radarc, tríd an gCiandracán = the view throughout the [Ciandracan] (this is a compound proper noun, "Cian" is "head" or "brain" and "racán" could be visor or rocket)
An Turas go Manannán = the Trip to Manannán
Manannán = Manannán (it's a noun, which is very similar to the Irish term for the Manx and the Isle of Mann).
Muintear Manannáin = the people of Manannán
na 'Cráidmí' = the Craidmi (I think it's just a plural noun)
An tÁrd-Máigistir = the high Magistrate, or possibly the supreme magistry
An Priorún = the Priory
Oidce sa Coill = The class/lesson/teaching in the woods/forest
An tinneall = the fire
Oidce tar Oidceanta = Lesson upon lesson
Lug Lám-fada = the long-armed lug
An Tróid leis na 'Cráidmí' = The war with the Craidmi
Diogaltas = Revenge
An téalod = not sure about this onethere are earlier north american examples; steam man of the prairies (1860s) being one.
(i'm big into mecha, sorry for the nitpick.)
It also looks like the PDF in the repo is just the first 20 pages, it would be worthwhile scanning the whole thing cleanly and uploading to preserve this important work.
While I am on mobile and (therefore) have not accessed the files, the ToC and description of the OCR process leads me to understand that the original print is in Irish, not English.