Hi Richard,
I came across your post while researching my own Nicholson ancestors from the same area and wanted to make contact.
I believe my ancestor Joseph Nicholson (born c. 1813, Ireland) emigrated to Warrick County, Indiana, where he died in 1878. His likely brother William Nicholson (born December 1815, County Kerry, Ireland) also settled in Warrick County and immigrated in 1841 according to his 1900 US census record.
Griffith's Valuation (c. 1852) lists Leonard, Thomas, and two Josephs (Senior and Junior) at Drimnamore, Kilcrohane civil parish — so Leonard and my Joseph appear to have been contemporaries in the same townland.
I also found in the IrishGenealogy.ie Kerry COI marriage records that a William Nicholson, George Nicholson, and Leonard Nicholson all married in February 1819 within ten days of each other — which suggests those three may have been brothers. Your Leonard matches the February 1819 record exactly.
I'm trying to identify who the father-generation Nicholson at Drimnamore was — the Tithe Applotment (1826) records a "Nicholson Junior" there, implying a Senior in the same townland at the same time. Any information you have about an earlier generation would be very welcome.
Thanks!