Ireland showed no signs of fatigue after their demolition of Scotland, as they recorded the second biggest win in their history to regain the trophy.
Joe Calcott scored Ireland's first two tries, scored in the seventh and 14th minutes, converting the second himself.
Wales hit back a few minutes later when Stuart Williams reached out well for a try that Scott Trigg-Turner improved.
But Ireland struck again with unconverted tries from Oran Spain and Mel Griffith to go into an 18-6 lead.
Griffith sailed over again soon after her first, with Johnston adding the goal, before a further Calcott try put the score onto 28-6 at the break.
Toby Burton-Carter increased Ireland's lead with an early second half try with Johnston converting before adding a try of his own.
And despite a few limited periods, Wales couldn't get back into the game, as the Irish completely took control.
Skipper Calcott added a further three tries and another goal, whilst Cian Horgan got a second half brace.
But the final word went to Wales with a Mark Williams try.