The actors who played
Merry and
Pippin used their own regional accents, they weren't "adopted" by them. And they weren't Irish. Dominic Monaghan is from Manchester, Billy Boyd is a Scot.
The fact that you percieved this as presenting the characters as "stupid" just confirms what I was saying previously about stereoypes and prejudices against regional accents, particularly northern accents. This is clearly YOUR prejudice, not Jackson's "shorthand". But thanks for proving my point.
As a northern English person, with a northern English accent, it's quite offensive to know that I would be percieved as stupid by you on the basis of my accent alone.