Wouldn't Lahinch and other Irish and Scottish courses be rated by Irish and Scottish golfers? Those guys are used to very strong winds and very F&F conditions that will mystify and terrify the average 12+ handicap coming from the US. So I'm not surprised at your claim that even in benign conditions that those guys are struggling. They are used to being able to hold the ball on the green, if they fly it on those greens it won't hold. They can't hit a high ball when the wind is blowing or they better be awfully good at guessing its effect. They aren't used to playing short game shots off extremely tight lies, or playing from deep pot bunkers. It is no wonder they do much worse than their handicap would suggest. Scratch golfers would do relatively better because they will have mastered more shots - even if they aren't used to the conditions at Lahinch, you can't be a one-dimensional player and reach scratch.
You'll find some Irish and Scottish golfers struggling on US courses that require have features they aren't used to, like being amongst trees as punishment for an offline drive, playing approaches from thick lush rough, approaches that require high carries with no allowance made for running it up, and thick rough around the greens that require mastery of flop shots with the LW.