I don't think a single referendum held on joining the EU has required a super majority. Can't think of one at least. Would be a much smaller union then, and Sweden wouldn't be members :-/
It has not required one, but the Union wasn't exactly an unpopular decision in many of the countries that did hold a referendum. If you required 60% for either side to win, the following countries would still have joined:
Ireland 83.1% in favour (10 May 1972)
Denmark 63.3% in favour (2 October 1972)
Austria 66.6% in favour (12 June 1994)
Slovenia 89.6% in favour (23 March 2003)
Hungary 83.7% in favour (12 April 2003)
Lithuania 89.9% in favour (1011 May 2003)
Slovakia 92.5% in favour, turnout 52.1% (1617 May 2003)
Poland 77.5% in favour (78 June 2003)
Czech Republic 77.3% in favour (1314 June 2003)
Estonia 66.8% in favour, turnout 64.1% (14 September 2003)
Latvia 67.0% in favour (20 September 2003)
Croatia 66.27% in favour (22 January 2012)
The Scandanivian countries were just more tricky.
Finland 56.9% in favour (16 October 1994)
Sweden 52.8% in favour (13 November 1994)
Norway 52.2% against (28 November 1994)
Malta 53.6% in favour (8 March 2003)
The original Brexit referendum in 1975 had a 2/3 majority even.