The US employee wouldn't be breaking any US law if they complied but might be breaking US law if they refused.
And here is the nub.
In the Google case, Google employees in the USA did have access as they controlled the servers and the data was moved purely for Google's convenience.
This case is very different. Microsoft have thought this through in advance, and setup a system which (it is claimed*) means that employees of Microsoft USA do not have access to the servers run by a legally separate company in Ireland. So the situation wasn't so much MS saying "we won't provide the data", it's more a case of "we cannot provide this data".
At best, MS USA can instruct MS in Ireland to send the data over - but that will get a fat rasperry in response because MS in Ireland will obey the local law.
Where it would get interesting is that if the DoJ win this case, a court could issue warrants for all the MS Ireland employees who have refused to hand over the data - effectively preventing them from ever travelling to the USA, or to any country "friendly" to the USA, on penalty of arrest and imprisonment.
* I have certain doubts that this is the case given that AIUI some of the authentication stuff can or is routed via US based servers (and certainly could be as it uses a domain name controlled from the US). Given that authentication can be routed via the US, it's hard to ignore the possibility of that being subverted to allow access.