In outline, Irish mythology (sources like the Lebor Gabála Érenn) says Ireland was occupied by the Túatha Dé Danann (a magical, beautiful, ethereal race), before the Milesians (Celts/men) took it. ‘Túatha’ is a word without English equivalent, meaning clan or tribe or petty kingdom; ‘Dé’ means ‘gods’; and Danann is the genitive form of Danu so it means ‘of Danu’.
Meanwhile, over in India, we read in the Mahābhārata that the goddess Danu gave birht to a race of asuras (demigods) called the Danava.
There’s lots of these little Indo-European links – in language, in culture, in myth. You see more and more of them the longer you look.
Also thought to be linked to the names of many great rivers. Danube, Dan, Don, etc.