He was appointed a justice of the
Court of King's Bench (Ireland) in 1798, and almost immediately afterwards he was appointed to the Special Commission established to deal with the aftermath of the
Irish Rebellion of 1798. In 1816 he presided at the much publicised
quo warranto case, ''Rex v. O'Grady''. In the same year, he presided at the celebrated trial for
murder of the Kerry
attorney, Rowan Cashel. Cashel, a notoriously quarrelsome young man, had killed his friend Henry Arthur O'Connor in a duel. He was acquitted, as was usual at the time in an affair of honour, after a charge to the
jury by Day in Cashel's favour. As a judge, Day was praised for his integrity, but not for his ability. Daniel O'Connell said that one could always win a case in front of Day by insisting on making the closing argument since Day, by his own admission, generally agreed with whoever spoke last (as Geoghegan remarks, many barristers still employ O'Connell's tactics). His colleague on the Irish Bench,
William Fletcher, was also given to treating grand juries to political harangues: his address to the grand jury of
County Wexford in 1814 caused uproar. His personal friendship with O'Connell even survived O'Connell's fatal
duel with John D'Esterre in January 1815. When Dublin was rife with news of the impending duel, Day was sent to arrest O'Connell, with the aim of preventing it. O'Connell insisted that he was not the aggressor in the matter, and Day, seemingly satisfied, merely
bound him over to keep the peace, thus making the death of D'Esterre inevitable. "Was there ever such a scene?" O'Connell asked later. Day lived long enough to welcome Catholic Emancipation, and was generous in his praise of O'Connell for his crucial role in achieving it. He retired from the Bench in 1818. He lived at
Merrion Square in Dublin city, at
Loughlinstown House in south County Dublin, and at Day Place, Tralee. ==Death, marriages and children==