Welcome to our service and thanks for your question. Let me introduce myself. My name is ***** ***** I will do whatever is possible to answer your question.
Since you've already checked on RootsinIreland, you know that no record has been transcribed for the births of any children to Francis o'Brien and ***** ***** in Co. Wexford. Nor is there any record of their marriage available online, or of the births of Francis and Catherine.
I checked passenger lists for 1855±2 years and did not find him. Of course, most passenger lists have not survived, and of those that have, most have never been transcribed and made available online.
If a record still exists for births/marriages it would be held locally in the church, and you would need to get the permission of the bishop to search the records. Alternately, you may find a record of wills, deeds, etc. in the courts in the area of Wexford where they lived, or copies in the National Library of Ireland. Griffith's Valuation (1849) is not available for Co. Wexford. It gave townland, parish and Barony and sometimes there were actual maps in some counties.
Certainly you will need to visit the Wexford Genealogy Center in person. I'm sorry they didn't respond to your written request. Perhaps they will be more responsive when you are there in person. It frequently takes 3-4 months to hear from them. You might want to give them a call and set up an appointment.
Are you familiar with Irish Times? http://www.irishtimes.com/ancestor/genie/ It is another for-fee service like RootsinIreland, but I've sometimes had some success using it for my private clients. I can't use it on Just Answer because I cannot pay the fees for a customer.
Please let me know if you have questions.