I am decended from a William Clark(e) born about 1721 and who may have come to Ireland from Bristol or Monmouth. He was probably the same William Clarke who was a tenant farmer at the Coarse Farm at Fethard on the Loftus estate of the Hook Peninsula in 1771 and who died in Fethard on the 7th May 1793 (gravestone visible in Fethard churchyard). His wife was Eliza was born about 1729 and is also buried in Fethard. She died 12th August 1787 according to the same gravestone.
William Clarke had a son, also called William, who was born about 1750, possibly in Bristol (from old family notes). He married a Miss Lett in 1790, possibly the same Margaret Clarke for whom there is also a gravestone in Fethard churchyard erected by her husband William Clarke (although he is not buried there as half the stone is blank and the intended inscription was never filled in). Margaret died on the 20th November 1823, and William was still farming in 1828 according to the Tithe Applotment book for Fethard.
William must have had at least one brother, as family records indicate that he had only two sons (1) Richard, born about 1792 and who was married in Waterford's Roman Catholic Trinity Cathedral on the 13th January 1817 where he is described as a gentleman of the parish of Fethard, County Wexford, and (2) Walter Rothe Clarke who was living in Waterford by 1841 before fleeing Ireland due to unpaid debts, and from whom I am decended.
The remaining Clarkes in the Fethard Tithe Applotment Book must therefore be decended from brothers to William Clarke junior. Your Thomas Clarke would sound likely as a brother or nephew to William Clarke junior.
During the 1798 Wexford uprising the brewer's son Philip Clarke 'a boy of about 13 years' and his mother were taken prisoner and sent to Scullabogue. He may have been a brother to your Thomas Clarke who was born about 1785.
I would be interested if you have more information, as it all helps piece together the tiny strands of information that have survived.
Best wishes
Phil Clarke