Not the most encouraging introduction in a language learning book.
Tá eagla orm.
FWIW I don't think there's any basis to this claim. And I didn't buy the book. Who would be confident with a teacher who says that something is a nightmare but not impossible?
@padraic Which book is this? IIRC Irish is considered a Category II Language for English speakers, on a par with German. It is nowhere as difficult as Japanese, Mandarin or Russian. It has some challenges with quite a complex grammar for an English speaker. And some of the irregular verbs are 'really' irregular, but there's only 11 of them. But pronunciation is very regular compared to English (-ough- words anybody?)
@padraic According to the FSI a category II language should take 828 class hours to learn compared to 2,200 class hours for a category IV language like Arabic or Korean.
https://www.state.gov/foreign-service-institute/foreign-language-training