Can I travel to European Countries with National Visa D?

With a valid Swiss D visa, you can travel in other Schengen countries as if you had a 90/180, multiple-entry Schengen visa.

You cannot travel to EU countries which are outside the Schengen area (e.g. Ireland, or currently the UK).
Days spent in Switzerland do not count for the 90/180 rule. Days spent in Schengen countries other than Switzerland count. The 90/180 rule applies over any combination of visa.
In those Schengen countries, you can do what any business traveller can do -- you can talk about working, but you are not allowed to start working (not even unpaid internships, and not even for a single day).

o.m. 34673

Source: stackoverflow.com