Oh, I found out why the plinth said that Queen Beatrix had been invited to do the unveiling: the Seven Dials area was developed during the reign of William and Mary, and Queen Beatrix was in the UK in 1989 to attend the celebrations of the 300^th anniversary of their accession to the throne. William of Orange-Nassau was a Dutch nobleman and a nephew of James II, and had married James' daughter Mary, so after James was deposed they were offered and accepted the joint monarchy of the UK.

It wasn't a happy marriage, unfortunately; William seemed to be far more interested in his boyfriends (two of whom he elevated to the English Peerage) than in his wife, and when he did have a relationship with a woman it was with his wife's Lady of the Bedchamber.

I wonder how many fervent Irish Unionists are aware that their beloved "King Billy" was a philandering queer :)