IMO it depends on the house.
I viewed a place in Rathmichael with a lovely, well-screened garden of about half an acre. The M50 noise was constant. I would have found it unbearable, because the garden was so lovely I'd want to spend time in it.
New build in Diswellstown with a postage stamp garden and same level of M50 noise - more a nuisance than a deal-killer.
edit: the Rathmichael one is still for sale over a year later. The BER cert is dates 29/04/2015, so it's been on for almost 2 years. I guess I'm not alone in being put off the noise at that price.
It is depends on the house, you right, and the price. Seen a house in Rathmichael, around 3 years ago, literally sitting on motorway, its back garden neighbouring with M50, so the noise was unbearable, for me, but house sold in less then 6 months. It was absolutely adorable house build in the wrong place. I'd say the distance to M50 was reflected in price, same house 2-3 miles away would be 2 times more expensive.