To get the ball rolling, here is an example of a Sudoku with 28 clues, and if my Sudoku generating program is right, none of them are redundant.
.1.|.6.|7.9
3..|...|...
.9.|...|...
---+---+---
7..|..8|4..
.4.|1..|...
9..|.4.|137
---+---+---
.7.|.86|9..
6..|5..|2..
.2.|4.7|.61
This is the unique solution.
412|863|759
387|954|612
596|721|384
---+---+---
761|398|425
243|175|896
958|642|137
---+---+---
175|286|943
634|519|278
829|437|561
My sudoku generator produces these only on the highest difficulty setting, which means that when you solve it you will have to some of the more advanced deduction techniques. There are likely to be unreducible sudokus with more clues, but which are solvable using only simple techniques.
Edit:
I have now looked through the book "Tak1ng Sud0ku Ser10usly", by Jason Rosenhouse and Laura Taalman. It says that the largest known number of clues is 39:
7.5|6..|8.4
64.|...|.27
128|47.|.56
---+---+---
251|.6.|..8
...|...|...
8..|.5.|26.
---+---+---
.8.|.3.|.7.
5.2|74.|.83
3.7|5..|4.2
It is not known (at least at the time of writing that book, 2011) whether there is an irreducible sudoku with 40 clues.