Hello, I just stumbled upon your post and found it interesting to note of your Cassidy family cattle dealers. Our Thomas Cassidy was a cattle dealer we suspect either Londonderry or Armagh. I too have found very little online except an article which speaks of the importance of cattle dealers. Ou Thomas was born about 1827 and married Margaret Somerville.
"The key figure in the success of the beef industry was the cattle dealer. He usually had some land of his own on which he could hold a stock of cattle. Like most other agricultural products cattle become ready for the market at the same time, namely at the end of summer, so a cattle dealer could hold over cattle to smooth out the supply to the English markets. As the cattle were bought and sold at the autumn fairs, sending them all directly to the English markets would cause a glut. The dealers had contacts with farmers, with the banks, with the railway and steamship companies and with the buyers at English fairs who were buying for the slaughterhouses. It was their organisation which made the beef industry profitable. The cattle from the less rich grasslands could not be sent directly to the butchers. The animals often had to finished off by fatteners who either had very good land or else stall-fed the cattle. Most of the cattle were shorthorns, except as noted above, Herefords in the Midlands. By the year 1900 many small farmers were using an Aberdeen Angus bull on small local cattle which produced a small animal which was popular with butchers. As with sheep and pigs, smaller and leaner animals were now being sought by the butchers. The whole tendency was away from enormous fat animals towards smaller animals with less fat. The drawback of this cross was that the cows were poor milkers (Farmers’ Gazette 9 June 1900)."