Ok, I think I've found the Guinness woman that James Keely married. I found a baptism record of Juila Kealy on irishgenealogy.ie named her father as James Keely, and the mother as Marianne, but no surname. I then found two certs for a Silvester Keely and Eliza Bridget Keely, both with father and mother listed as James and Mary Anne. Eliza's female sponsor is a Bridget Guinness, and Silvester's male sponsor is a Richard Guinness. They are all in St. Andrew's church (a.k.a. Westland Row). It all fits nicely so far as Silvester (or Sylvester) is the name of one of Thomas Duggan and Julia Keely's sons.
Having looked through google for information on the Guinness family tree, it's all a bit confusing. I can only see one grandchild of Richard Guinness named Mary Anne, apparently Arthur's own daughter, who seems to have married a man named Burke. I'm sceptical of these links as I don't see any proof, so I think I'm going to go into The National Library tomorrow and just scour through the parish records for St. Andrews for the time. The most important cert I shall be looking for will be the marriage of James Keely and Mary Anne, which should hopefully name her father. The three baptism records I found were from 1830 to 1833 so I think I'll just go backwards from 1830 and hope for the best....