RootsChat.Com

I'm pretty confident that Julia's father, Michael, was the son of a William Britt who married Mary Lanigan and raised their family in Pottlerath, Kilmanagh about 10 miles to the west of Kilkenny. They had a son Michael who was born in 1833, but I had no idea what happened to him - presumably he moved to Kilkenny.
Some of the Britts of Pottlerath definitely moved to Blackmill, and many of them did go from there to the US. One of the letters you posted mentions the Lanigans and Brennans of Blackmill, and one of William and Mary's daughters did marry a Brennan. By 1901 there were no longer any Britts living in Pottlerath.
If you give me some further details (perhaps by personal message) of who Julia and her sisters married, where they lived etc., then I think I should be able to piece it all together. Also, where did Julia and Thomas live in the US? Any clues at all might help.
Thomas Britt (who married Johanna Kelly) was originally from Pottlerath but later moved to Kilkenny. This Thomas was a nephew of the William I mentioned at the start.

I had the same problem with the civil parish of Kilmanagh in Co. Kilkenny - the numbers did not match up with the plots that were shown on the corresponding maps.
However, when I viewed the Valuation Books from later years at the Valuation Office in Dublin, I found that the numbers had been changed by about 1862 so that they matched the plots on the map properly.
Jud

I hadn't noticed that! Even entering just one first name does give results in advanced search.

Having used the rootsireland site for a number of years, I finally realised today that it is possible to make ADVANCED searches without entering anything in the SURNAME* box

For example, by simply entering the mother's surname as Delahunty, it is possible to find all the baptism records in a particular parish which have the MOTHER'S SURNAME recorded as Delahunty.

This also works for marriage records - you can get all the marriages where the mother of either the bride or groom was recorded as Delahunty.

This may already be known to many, but it has lead to a few new 'discoveries' for me today, so am happy to broadcast it. 

The church records of most of the parishes in South Tipperary are not available in the National Library, and are not available on www.rootsireland.ie .
The only way to access these records seems to be by getting the South Tipperary Family History Centre to conduct very expensive searches of their archives.
In the National Library, I was told that I should write to the appropriate diocese to get permission to view the records, but when I did this they responded by saying that I should re-direct my enquiries to the South Tipperary Family History Centre.
I have only come across this problem in South Tipperary, and would love for someone to explain to me the reason or logic behind this.
Also, are there other counties where this happens?

So, Marcella, I think there are some cases where difficulties do occur, but it is not the norm. I would recommend that you use the free-search facility on the rootsireland website to identify in which parishes and in which years you are likely to find the records that you are looking for.