Thanks heywood I had a look at all the links for clues
There are several possibilities for the birth of
Catherine KELLY and
Stephen POWER in the correct time frame for marrying in 1906; i.e. born between 1876 (age 30) and 1888 (age 18).
There are several you can rule out immediately as they also have either a baptism record that shows the parent's names or a corresponding death; you can also rule out some by going through the census records to find matches.
You will see that there are far more Catherine KELLY's than Stephen POWER's!
It looks like a big job to go through them all.
If the family moved around quite a bit, there may be no records to help confirm relationships for people. I know that you have the 1906 marriage certificate for Catherine KELLY and Stephen POWER, the church records at that date will not be online but may hold additional information especially if the couple were not resident in the parish. The priest may have added a note to the register about their origins or family.
There are some other ways of locating people e.g. the workhouse or prison records but sometimes you have to work from initials which does not help confirm information
There are a few prison records for a Stephen Power and a Charles Kelly in Wexford but unfortunately they do not show a next of kin; you would have to go through each on its merit and try and rule it out or in with other information.
There are a few Stephen Powers on the 1901 census living with widowed mothers, two are in Waterford, that may be your Stephen but again without prooving the father was not just Michael but the correct Michael you'll be back to square one.
Best of luck in the search