Ngayon, Kukulen ay naging tanda.  => Ngayon, si Kukulen ay nasa hustong gulang (or edad) na. =  Now, Kukulen is at the right/ripe age.

Gusto niya maging kasado sa imer, ang anak na babae ni ‘Forgal ang tuso’.  => Gusto niyang makasal kay Imer, ang anak na babae ni “Forgal ang tuso”. = He wants to be married to Imer, the daughter of “Forgal the wily/shrewd”.  (“Tuso” is a rarely used term. It’s a negative trait, like describing someone who’d pull the rug from under you). 

Noong alam ni Forgal nga, sabi niya sa Haring Konor tungkol ang galing ng Cuchullain. (not sure of this)  => Nang malaman (nga) (“nga” may be placed there, but not really needed) ito ni Forgal, sinabi niya kay Haring Konor ang tungkol sa galing ni Cuchullain. =  When Forgal learned of this, he told King Konor about the skillfullness of Cuchullain.  (Are Kukulen and Cuchullain just one and the same person? If yes, then the idea of this entire sentence would not be right.  What is King Konor’s relation to Kukulen?)

Sabi niya rin dapat ni Haring Konor magpadala ni Kukulen sa isang isla tabi ng banyang Scotland, kung saan dapat bihasa si kukulen ni Skahak. (not sure of this one) => Sabi niya rin na dapat ipadala ni Haring Konor si Kukulen sa isang isla na malapit sa bansang Scotland kung saan dapat mabihag ni Kukulen si Skahak. = He also said that King Konor should send Kukulen to an island near Scotland where Kukulen must be able to capture Skahak.  (“Katabi” when used in this sense means “adjoining” more than “adjacent”.  Better to use “malapit” to mean near.  “Bihasa” is “accustomed to” and does not seem to make sense as used in the sentence.  Hence, I thought that maybe “mabihag” (to capture) is what you meant.  I am not sure though whether it’s Kukulen or Skahak who’s supposed to be captured.)       

Babaeng mandirigmang mabangis ay Skahak, kaya sana ni Forgal magtanggal ng CuChullain sa buhay ng kanyang anak, at alam niya marami ng mga tao kung sino magsanay doon ay hindi matirang buhay. => Babaeng mandirigma at mabangis si Skahak, kaya paraan sana ito ni Forgal na matanggal si Cuchullain sa buhay ng kanyang anak kasi alam niya na sa dami/dinami ng taong sumubok doon ay walang natirang buhay. = Skahak is a female warrior and ferocious/a ferocious female warrior, that Forgal hopes this could be his way to take Cuchullain out of his daughter’s life because he knows that with so many who had tried/made the attempt there, no one survived. (“Magsanay” is “to practice”.  I don’t think that’s what you meant.)     

(Let me know if my interpretation is correct so far.  The 3^rd sentence does not seem to fit well into the flow of the story.)

Neil, I almost forgot that we still have our unfinished business here.  :)

Natapos niya ang kanyang pagsasanay dahil sa kagalingan niya.  =  He was able to complete his training as a result of his skillfulness. (“Tinapos niya” = he completed, “natapos niya” = he was able to complete.  Though both may mean the same thing, the latter adds a sense of exerting effort in its completion, which I think is the idea you wanted to convey.)

Bilang pambayad/pang bayad sa kanyang pagsasany, kinailangan ni Kukuleng/Kukulen na gumawa ng isang bagay para kay Skahak.  =  As payment for his training, it was necessary for Kukulen to do one thing for Skahak.  (“Para pampayad” is “for the payment of”, which is not really wrong as you used it in your sentence.  However, using “bilang” would give the “to serve as payment” or “as a way of paying” meaning instead.  Use “bagay” (a thing) so as not to use both “gumawa” and “gawain” that close to each other in a sentence.  That’s because to do so would be like saying, “to work on a piece of work”.    

Sinabihan niya si Kukulen na dapat talunin nito/niya ang isang mabangis/matapang na babaeng mandirigma na ang pangalan ay Eefa.  =  He told Kukulen that he must defeat a fierce female warrior named Eefa.  (Talúnin – said slowly, with accent on 2^nd syllable, means “to defeat”.  “Talunin” said fast, with no accent, means “to jump over”.)

Hinanap ni Kukulen si Eefa, at ang kinawakasan/katapusan nito ay tinalo niya si Eefa at kinuha niya ang balitang harpon (?) na/harpong Gae Bulga.  = Kukulen searched for Eefa, and the ending of this was that he defeated Eefa and he took the legendary/well-known ??? Gae Bulga.  (“Kinatapusan nito” or “kinawakasan nito” means “the ending of this”.   “Sa wakas” means “finally”, but suggests success only after repeated attempts or after a period of time.  “Sa wakas at dumating din si Maria.” = Finally, Maria arrived (after we’ve been waiting for her for so long).

Tapos, pinanganak ang kanilang sanggol na si Conlak. = Then, Conlak, their infant/baby/child was born.  (Seems like you fast-forwarded the story that I got lost as to whom Kukulen ended up having a baby with.  Was it with Eefa?)   

Umalis si Kukulen sa isla ni Skahak kasama dalawa ng kanyang kaibigan. => Pumunta si Kukulen sa isla ni Skahak kasama ang dalawa niyang kaibigan (or kasama ang dalawa sa kanyang mga kaibigan.) = Kukulen went to the island of Skahak together with 2 of his friends. (“Umalis si Kukulen papunta/at pumunta sa isla ni Skahak...” = Kukulen left to go/and went to the island of Skahak...

Matagal at mahirap ang kanyang pupuntahan. => Matagal at mahirap ang kanilang pagpunta doon. = Their way there was long and difficult. (Use “kanilang” because he is with 2 of his friends).

Sa wakas, nakarating siya ang kuta ng Skahak at nanakot siya ni Skahak hangang naging siya ang kanyang estudyante. (“nanakot” is to frighten, intimidate). => Sa wakas, nakarating sila sa kuta ni Skahak at tinakot ni Kukulen (niya) si Skahak hanggang siya ay naging estudyante nito. = Finally, they arrived at Skahak’s fort and Kukulen (he) intimidated Skahak till he became her student.

May gandang iha ni Skahak at nagkasintahan si Cuchullain niya habang pag-aral niya. = May magandang anak (na babae) si Skahak at ito/siya ay naging kasintahan ni Cuchullian habang siya ay nag-aaral. = Skahak has a beautiful daughter and she became Cuchullian’s sweetheart while he was studying. (“Iha” is from the Spanish “hija” (daughter). Nowadays, it is used by some of the elders as an endearing address to a young girl, normally, young enough to be their grand-daughter. If said to someone older, it is usually intended as a disparagement, i.e., that one is not wise enough for her age. Also, because the Tagalog 3rd person pronouns have no gender, the “siya ay nag-aaral” may also be understood to refer to Skahak’s daughter, though unlikely, based on context.)