Clunsast/Clonsast Upper and Lower are 1,077 and 1,775 acre townlands in Co. Offaly. The parish is likely Edenderry. There are NO B rcds for children with a father forenamed Thomas in Co. Offaly earlier than 1825. All are outside your timeline.
This infers 1) The records didn't survive (most likely). 2) They were born elsewhere. Or 3) The records haven't been transcribed (a very slim chance).