↑ Patota, Giuseppe(2002)Lineamenti di grammatica storica dell'italiano(in Italian), Bologna: il Mulino, ISBN 88-15-08638-2, page 146
Etymology
From Latin-īverunt (via -īru). In the thirteenth century, the Italian form was -iro, which later was extended to -irono under influence of verb endings like -ano and -ono.[1]
Example: Italian finirono, from Latin finiverunt.