tricky. it can be argued that nationalism is inherently authoritarian towards other countries/ethnic groups. but although most nationalist governments have been deeply authoritarian towards their own people this seems to stem from other ideological arguments espoused by the governments (fascism/nazism/socialism etc.)
The SNP and Plaid Cymru (for example) are definitely not authoritarian, but it is arguable that they are not properly "nationalist" - so it can become a bit of a circular argument.
And of course, the Unionist have always been such a fine upstanding example of decorum and restraint, haven't they? If I were looking for an example of authoritarism and nationalism, even now with their numbers dwindling and the modern world slowly but surely catching up to them, they still expect the rest of us to dance to their tune.