Many Irish people dislike Synge because they regard the language he uses in his plays as an unnatural pastiche, a gimmick which worked for him in his first play, and which he subsequently milked dry. It bears no relation to the way people actually spoke, making it hard to view the plays as any kind of accurate representation of life in the region.
The basic charge, and one which I feel is completely accurate, is that he was nothing more than a 'faker of peasant speech'. That audiences were only too willing to lap up the kind of paddywhackery that he peddled doesn't make it any less despicable.
"MAURYA (Continues without hearing anything.): There was Sheamus and his father, and his own father again, were lost in a dark night, and not a stick or sign was seen of them when the sun went up. There was Patch after was drowned out of a curagh that turned over. I was sitting here with Bartley, and he a baby, lying on my two knees, and I seen two women, and three women, and four women coming in, and they crossing themselves, and not saying a word. I looked out then, and there were men coming after them, and they holding a thing in the half of a red sail, and water dripping out of it -- it was a dry day, Nora -- and leaving a track to the door." - J. M. Synge, 'Riders to the Sea'.