Milestone man Dacey is one of seven changes made by Director of Rugby Mark Hammett as Blues look for a return to winning ways at the home of the reigning league champions in Dublin.
Wales international 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 while flanker Sam Warburton returns 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 while 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 hooker Matthew Rees, back row Manoa Vosawai and scrum half Tavis Knoyle who could also make his league debut for Blues. Lock Filo Paulo and centre Gavin Evans both miss the game through injury.
“Leinster are the champions for a reason and they have an extremely strong side and very powerful squad,” said Hammett.
“We are all certainly looking forward to the challenge that we will face over in Dublin. The atmosphere will be big over there and we got have to be ready for that.
“We are working exceptionally hard every week in training to improve and I felt our attack was much better last week against Ulster.
“But we are not finishing our chances when they are there, and we certainly created some good opportunities and drew penalties last week. We now need to get across the line and show that clinical edge against a well drilled and talented Leinster side.”
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