Hello Kathy,
If you were to consider some of the other information from the marriage and baptismal records that you have it could open up other avenues to you. Often the people who acted as witnesses at weddings or sponsors at baptisms were other family members or close neighbours. Do you have many records for Peter and Jane’s children?
I see that one of the marriage witnesses is a Hanna Sweeny. I had a quick look for baptisms of Jane Brennan in Rathfarnham and found this.
http://churchrecords.irishgenealogy.ie/churchrecords/details/8f868e0027487
Notice the sponsors names are Sweeney. You could try searching for other records pertaining to the father’s name Mic (Michael) and wife Eliza.
I found this- http://churchrecords.irishgenealogy.ie/churchrecords/details/6818ca0392767
I am not saying that these are the next generation that you seek, but I would certainly think that they are worth investigating from the information in your message. Maybe some of it will chime with research you have done already.
To find out other information you could try checking the land records called the Tithe Applotment Books (1823-38) http://titheapplotmentbooks.nationalarchives.ie/search/tab/home.jsp or the later Griffith's Valuation (1848-64) http://www.askaboutireland.ie/griffith-valuation/
The Tithe Applotment List might be of use to you, or at least interesting for you. These lists constitute the only nationwide survey for the period, and are valuable because the heaviest burden of the tithes to the Established Church, the Church of Ireland, fell on the poorest, for whom few other records survive. The information in the Tithes is quite basic, typically consisting of townland name, landholder's name, area of land and tithes payable. Many Books also record the landlord's name and an assessment of the economic productivity of the land. The tax payable was based on the average price of wheat and oats over the seven years up to 1823, and was levied at a different rate depending on the quality of land. For Parishes where the registers do not begin until after 1850, this information can be useful, as they are often the only surviving early records. They can provide valuable circumstantial evidence, especially where a holding passed from father to son in the period between the Tithe survey and Griffith's Valuation.
Best regards Michael.