M6 - no real impact to be honest. It's too far away from the airport. In time, if the M17 is ever built to Tuam that could have an impact in bringing Galway City more into the airport's range, but it would also make SNN more accessible to users of NOC. So potentially both positive and negative I suppose.
Would be nice to have a big city next to Knock but there isn't one so Knock survives by being central to a wide geographical area of around half a million people who are quite spread out, rather than one focus city like a lot of other airports.