Colorful expression conveying extreme astonishment/incredulity. Context: Speaker is a ship's cook having just encountered a mixed group of humans and non-humans, one of whom has just announced that they intend to save the world from alien invasion.

As this is for a YA novel, a not-too-profane expression would suit. In the States it would have a movie rating of PG-13 or tamer. If there's something a bit salty that would slip past English-only editors and movie censors, that would be fun.

/Thanks!

Here are some fairly innocuous possibilities:

A thiarcais!
My goodness!

Ar m’anam! or Dar m’anam!
[On] my soul!

Ar m’fhocal! or Dar m’fhocal!
[On] my word!

Dar Chroim!
By Jove [actually, "By Crom", Crom being an ancient Celtic God]

Dar Fia! or Dia Fia!
By Jove! [Fia was apparently another ancient Celtic God]

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I'm not a native (or entirely fluent) speaker, so be sure to wait for confirmations/corrections, especially for tattoos.

Thanks! Definitely getting there, but I'd like to crank it up a bit. At one point another character (of Swedish heritage) claims that something is "a crock of skit", the last word being Swedish for what it rhymes with in EN, differing only at the 2nd letter. It's funny, not too naughty, and gets past puzzled censors who don't know any better.

Anything that's more along the lines of WTF?

/Thanx again!

Traditionally, Irish could be very earthy, so one could say things like "rud caca" ("a worthless thing" - literally, "a shitty thing"), but curses and ejaculations were more religiously oriented than scatological, and the devil featured fairly strongly. One possibility as an earthier form of ejaculation is "M'anam don diabhal!" (literally: "My soul to the devil", but more figuratively: "Well, I'll be damned!").

_________________
I'm not a native (or entirely fluent) speaker, so be sure to wait for confirmations/corrections, especially for tattoos.

Perfect! As this character is Irish Catholic, it fits well.

/Thanks!