Castlebar's first extant charter is dated 1614, 11 James I. It provided for a portreeve, 15 free burgesses and a commonalty, and its provisions were said to have been observed until the Union when the borough was disfranchised. Throughout the eighteenth century it belonged to the Bingham family, at least one of whom sat for it from 1692 to 1743 (two from 1715 to 1727). After 1742 they tended to sit for the county and - as in 1761, when Sir Charles Bingham was returned for both the county and Castlebar - to use the borough as a safety net. In 1783 the town consisted of '13 Burgesses, non-resident. 447 Houses, 145 Protestant Householders. Patron, Lord Lucan (Sir Charles created 1776, Lord Lucan) 4,230 inhabitants.'
In 1790 it was described as 'a close Borough, its electors consisting of a small number of Burgesses only, who are entirely under the dominion of the Bingham family, Lord Lucan's patronage invariably fixing them in their stations and his pleasure determining their nominal choice of representatives.' At the same time it was considered that as Lord Lucan since 1781:
has constantly resided in England, this Borough has been regularly exposed to sale and become the temporary property of the highest bidders. Its price increasing that destructive drain of cash which the absentees of Ireland suck from the very vitals of the community. Its lot at the next general election will, in a great measure, depend on the issue of the contest for the County.299
In 1791 it was reported that 'He sold the seats to the Bishop of Limerick, who gave them to Government, for his Peerage (Lord Glentworth, 1790).'300 At the Union Castlebar was disfranchised and the £15,000 compensation awarded to Richard, 2nd Earl of Lucan - Sir Charles died in 1799.