Mainly the no voters were and are farmers and fishermen, the politcal left of leftoffcenter, and nasionalist.
Much the same as here in Finland. I think in both countries membership was opposed by those who saw it as a political/economic elite project that would lead to losing national sovereignty to "faceless Eurocrats". Basically all those who were against "centralization of power" and foreign/international business interests anyway.
Here, this included the (pro-Russian) far left, the independent-minded nationalists, and the Center Party. The last is the most important. The CP is and was a countryside party: the majority of its voters are farmers, live in the rural areas/smaller communities or are originally from those areas. It had and has the most anti-EU voters. Of the big three parties, it was the one that could have torpedoed the Finnish membership.
But luckily (?), in 1991-1995 it was the leading party in government, led by the new-generation economic liberal, Esko Aho. Aho and his allies did everything they could to get Finland into the Union, including going against the majority opinion of his own constituents. For example, it has been said that the Finnish government deliberately hastened the EU referendum here to hold it a month before the Norwegian one, in October 1994, because it feared that the Norwegians reject the membership and that would be a negative influence on the Finnish voters.
Had the CP been in opposition, led by an old guard guy like, say, Väyrynen, the anti-EU campaign might have been much more intense and Finland might well have followed the Norwegian model.