He entered Trinity College Dublin on 12 February 1752, but did not graduate and instead joined the British Army.
[2] Between 1758 and 1776, Jeffereyes sat in the Irish House of Commons as the Member of Parliament for Midleton, before representing Randalstown from 1776 until his death in 1780.
In 1778 Jeffreys supported the popery bill granting Irish Roman Catholics greater property rights; Shannon opposed it.
[4] During the 1760s and 1770s he spent over £8,000 as loans to manufacturers or in building premises on the estate which he then leased out to encourage economic activity.
He invested in improvement works, including housing for workers and water power for local industry.