Tim:
Welcome to Ireland Reaching Out!
There is a gap in the Castlecomer RC baptismal records https://registers.nli.ie/parishes/0981 from 1818-1828 and very likely Daniel and other older siblings of William were caught in that gap and you won't locate any records. Marriage records did not start until 1831.
I also looked at the 1823 Tithe listings for Castlecomer parish and found a William Farrell with a very small holding (less than an acre) and a Michael Farrell who had four acres both living in Moneenroe. Here is a link to the Tithe record for Michael. Not sure how Michael (and William) are connected to the older Daniel. By the way, Moneenroe was a large townland with over 100 households.
http://titheapplotmentbooks.nationalarchives.ie/reels/tab//004587414/00…
I also looked at the 1849 Griffiths Valuation head of household listing for Castlecomer civil parish and although there were Farrell records, there was no Daniel and no one in Moneenroe. By the 1901 census there were no Farrells in Moneenroe but there were Bergins in the census.
http://www.failteromhat.com/griffiths/kilkenny/castlecomer.htm
You may want to consider autosomal DNA testing.
Also, please consider adding the stories of William and Daniel to our XO Chronicles site https://www.irelandxo.com/ireland-xo/history-and-genealogy/ancestor-dat…
Let me know if you have any questions.
Roger McDonnell