MacDonald Dress Modern is an asymmetric tartan but the MacD Dress Irish is not asymmetric. I also wouldn't pleat this to anything but the sett. I usually decide on pleating by finding the most prominant stripe, in this case it is the white stripe. But I wouldn't choose the white stipe becase of the width and dominance of the vertical stripe that washes out all but the white.

Pleating to blue might be nice but you will definitely get the horizontal "lawn chair" effect where the white horizontal stripes will dominate the pleats. Therefore, I would pleat to the sett.

Of course, I do like pleating to a none dominant stripe like the blue in a MacPherson, so do some trial pleating by pinning and see what you like.

Cheers,