[1] Henry was the fourth son of Samuel Ussher, rector of Dunganstown, County Wicklow, by his wife Frances Walsh.
Appointed on 22 Jan 1783, the first Andrews Professor of Astronomy, Ussher went to London to order instruments for the planned Dunsink Observatory from Jesse Ramsden.
These included: a small achromatic lens telescope, mounted on a polar axis, and carried by a heliostatic movement; an equatorial machine with circles five feet in diameter; a transit of six feet focal length, and a ten-foot vertical circle executed, after delays, on a reduced scale.
Ussher chose the site for the observatory at Dunsink near Dublin, planned the building, and supervised its construction.
[2] Papers contributed by Ussher to the Transactions of the Royal Irish Academy included "Observations on the Disappearance and Reappearance of Saturn's Rings in the Year 1789".