The commutation factor can depend on the rules of the scheme (if it's a occupational pension scheme) or can depend on the current annuity rate - which will vary by sex, age, type of annuity, etc - I don't think that the Revenue have a fixed rate.
the only time I have heard of a Revenue factor for commutation is in relation to maximum benefits introduced in the last finance act. 5 million is maximum fund amount you can have and afaik they use a factor of 20 to value the pension amount in a defined benefit scheme. in a defined contribution scheme the value is obvious.
For commutation factors in a regular defined benefit scheme, they are usually scheme specific and advised by the Actuary and agreed by the trustees.
For what purpose are you looking for the factor...if it is for a small self-admin scheme, it will need to be on an "open market" basis but if it is for a large occupational scheme then it will usually be scheme specific - as advised by the scheme's actuary for a defined benefit scheme.