PRICES are not an issue for Pimlico Opera's visit to DCU this day week. Its performances of Mozart's Don Giovanni (2 p.m. for children, 7.30 p.m. for adults) are being given free for the people of Ballymun. If you're not from Ballymun, you can pay for your Mozart at Glin Castle in Co Limerick (Saturday 27th), Birr Castle (Sunday 28th), Tullynally Castle in Castlepollard (Tuesday 30th and Wednesday 31st), or Derry (Friday, August 2nd) January 17th next has been set as the date for the opening concert by the Ulster Orchestra in Belfast's new £29 million sterling Waterfront Hall with James Galway. Barry Douglas and Polish conductor Jerzy Maksymiuk Before that, the orchestra's incoming principal conductor, Dmitri Sitkovetsky, opens his first season at the Ulster Hall on September 27th Departure wise one of the great horn players of the century. Barry Tuckwell (65), plays with the Orchestra of St Cecilia at the NCH tomorrow for what will probably be his last appearance as a soloist in Ireland he is advertised as making his London farewell at the Proms next month.