Milestone man Dacey is one of seven changes made by Director of Rugby Mark Hammett as Blues look for a return to winning ways in Dublin.
Prop Gethin Jenkins will make his first league appearance of the season alongside Dacey in the front row.
Two further call-ups in the pack see lock Chris Dicomidis set for his first start of the campaign and flanker Sam Warburton return in the back row. Josh Navidi moves to Number Eight and will captain the team.
Centres Cory Allen and Dafydd Hewitt are both named in the starting line-up and winger Watkins is rewarded for his stunning recent form for Cardiff RFC, where he has scored five tries in two games, with a start out wide.
A powerful bench includes scrum half Tavis Knoyle who could make his league debut for Blues.
"Leinster are the champions with an extremely strong side and have a very powerful squad," said Hammett. "We are certainly looking forward to the challenge that we will face over in Dublin. The atmosphere will be big over there and we have to be ready for that.
"Last week we had a short turnaround between games and maybe ran out of steam towards the end against Ulster. We gave it at an honest and tough second 40 minutes but probably fell away a little bit on that hard track. Now we need to be refreshed and ready to go.
"We are working exceptionally hard every week in training to improve and I felt our attack was much better last week. But we are not finishing out chances when they are there and we created some good opportunities and drew penalties. We need to get across the line."
Cardiff Blues: 15 Rhys Patchell, 14 Alex Cuthbert, 13 Cory Allen, 12 Dafydd Hewitt, 11 George Watkins, 10 Gareth Davies, 9 Lewis Jones; 1 Gethin Jenkins, 2 Kristian Dacey, 3 Adam Jones, 4 Jarrad Hoeata, 5 Chris Dicomidis, 6 Josh Turnbull, 7 Sam Warburton, 8 Josh Navidi (c)
Replacements: 16 Matthew Rees, 17 Sam Hobbs, 18 Taufa'ao Filise, 19 Macauley Cook, 20 Manoa Vosawai, 21Tavis Knoyle, 22 Adam Thomas, 23 Dan Fish
Guinness PRO12
Suite 208, Alexandra House,
The Sweepstakes
Ballsbridge, Dublin 4, Ireland