Answer:
after
Step-by-step explanation:
A subordinating conjunction is a word that connects an independent clause with a dependent clause.
"After" is the word that connects the independent clause (the part of the sentence that stands on its own)
"Celeste gets to visit Ireland"
with the dependent clause (the part of the sentence that needs the other part of the sentence, it's dependent on it)
"the semester is over."
Celeste gets to visit Ireland after the semester is over.
hope this helped :)