Co-promoted by BBC Northern Ireland and RTE, two shows will take place simultaneously outside Belfast City Hall and, subject to licence, Dublin's O'Connell Street.
25,000 tickets are being made available by online and phone ballot – log on to bbc or rte or call 0901 293 2004 (The North) and 1570 717 000 (The South) before the end of Friday March 12 for your chance to get a pair of tickets from your preferred city.
Proceeds from the premium rate lines go to BBC Children In Need and, in the Republic, to People In Need.
It's Morissette's first Irish appearance since her now legendary 2001 club show in Vicar St. Since then she's recorded a new album, So-Called Chaos, that's due on May 18 and had several feet lopped off her mane.