Following the succession of James II the English throne in February 1685, the army in Ireland was to see some very drastic reforms and reorganisations. One of these reforms was the purge of Protestants from the Irish army, and filling the places with Catholics. Another was the disbandment of troop of life guards on the Irish Establishment, probably related to the aforementioned purge of Protestants.
This troop of life guards was raised in 1662 and contained a troop of horse grenadiers in 1685. As said, this troop of life guards was disbanded (in July 1685), but from the Calendar of State Papers Domestic : James II it becomes clear that the horse grenadiers were retained.
Initially it was without a captain, but on 4 November 1685 Laurence Dempsey was appointed as captain of this troop of horse grenadiers. He was succeeded on 1 March 1686 by John Salkeld. Salkeld was replaced on 30 June 1686 by Pierce (Butler), 2nd Viscount Ikerrin.
It is, however, not sure if the correct Viscount Ikerrin is considered. Though the CSPD : James II explicitly mention Pierce, Viscount Ikerrin as captain there are some caveats. First, this 2nd Viscount Ikerrin seem to have adhered to the established church, which seems to contradict the purge of Protestants in favour of Catholics. Secondly, some sources indicate that the 2nd Viscount had deceased already by 1680!
Thus, it may very well be that James (Butler), 3rd Viscount Ikerrin became the captain of the troop of grenadiers in 1686.
The fate of the troop after 1688 is unsure. In James' Irish army of 1689 we find a troop of horse grenadiers, attached to the two troops of life guards. This troop was commanded by a Colonel Butler, of which no more information could be established. Because of this Butler, there might be some continuity between the pre-1688 and post-1688 troop of horse grenadiers.