Well there will be bad news and also some good news:
***first of all, the actor is definitly Irish and obviously has Irish accent which is a great thing as you already said. But the age? I am not worried about that much! its not 2000s is it? Now days you can see many many young actors or actresses at this age playing great. For example remember the movie Room playing by the Jacob Tremblay who was "9" in 2015. Or the Netflix's new series Stranger thing where u can find Millie Bobby Brown age 13 and Noah Schnapp 12 and they are playing pretty well (and many more movies even Logan!) I agree that playing the character Artemis Fowl is actually the most important, kinda difficult and crusial role in the movie to impress people but still I think they CAN find THE ONE. I hope so!
And remember if they decide to make more AF movies (THEY GONNA) just imagine how mature and old Artemis gonna be in 2nd movie (or 3rd even).
***The actor is going to be a new talent which means no experience. The bright side is nobody knows him, so he can be like a daniel radcliffe as in Harry potter. But the dark side though... You know
***He is not gonna be black trust me. or Asian or Latin or any of them. Its Ireland
***THIS IS WHAT I CAN NOT IMAGINE IN THE MOVIE : "The Wexford author previously said he would like Saoirse Ronan to play one of the lead roles in the big-screen adaptation."
lead roles means Holly Short right? I cant picture her as Holly Short :| no way! Opal Koboi? never she doesn't look kinda evil! Juliet Butler? she is too old for that. so..
***My huge concern now is script. I dont know how they gonna make 1st and 2nd book in one movie. There is only one way and that is making a film long enough to cover every "important" detail. something like 2h30m (or even more) But we never know maybe McPherson is a genius.
But totally I trust disney. they ready to pay, the dont play around and they are serious. Kenneth Branagh is not THAT high profile DIRECTOR but still I believe he can do it. Harvey and Robert De Niro are another huge names in this upcoming movie and as I know Lucy Bevan as casting director Artemis gonna be fine
And definitly we gonna see more famous faces in this GONNA BE GREAT movie!