I think the absence of an Ireland situation for Switzerland is meaningful, here. There isn't a preexisting open border agreement that Switzerland is obligated to uphold by both the EU and the US, which the Good Friday Agreement is for the UK. That open border agreement also seems to go further than the Schengen agreement in practice considering Switzerland already has regular (but not universal) customs checks due to its ex-EEA status, not to mention other Schengen member nations such as Denmark which have at times maintained consistent immigration checks.