Short answers, pulled from the story.
Michael Kelly (the 25th of December 1762 - the 9th of October 1826) was an Irish tenor, composer, and theatrical manager who built an international career in the late eighteenth and early nineteenth centuries. He created roles in operas by Mozart, Paisiello, and Storace, and was one of the first tenors from Britain and Ireland to achieve fame in Italy and Austria.
Michael Kelly sang two roles in the 1786 premiere of Mozart's The Marriage of Figaro: Don Curzio, the stuttering lawyer, and Don Basilio. He famously argued with Mozart over whether to stutter in the concerted ensembles, insisted on doing so, and Mozart was ultimately greatly satisfied with the result.
Kelly was on close personal terms with Wolfgang and Constanze Mozart during his years in Vienna from 1783 to 1787. He dined with Mozart regularly, lost to him repeatedly at billiards, and was present when Mozart played through the duet "Crudel, perche finora?" with the ink still wet. The two parted in tears of friendship when Kelly left Vienna in February 1787.
Kelly trained in Dublin under the Italian singers Passerini and Niccolo Peretti, and later under Michael Arne and Venanzio Rauzzini. In 1779, he enrolled at the Conservatorio Santa Maria di Loreto in Naples under Fenaroli, and received intensive private tuition from the male soprano Giuseppe Aprile (1732-1813) in Gaeta and Palermo, during which his voice settled into a tenor range.
Kelly was the principal English-language tenor at Theatre Royal, Drury Lane from his debut there in 1787. He performed there through the close of the old building on the 4th of June 1791, and continued at the venue and at the King's Theatre, where he became acting manager in 1793.
Kelly published his Reminiscences in 1826, written with the assistance of Theodore Hook, as the primary account of his life and career. Scholars have questioned the book's accuracy: one assessment states that any statement of Kelly's is immediately suspect. The Reminiscences remain the main firsthand source for his accounts of Mozart, Salieri, Paisiello, and others.