The singer-songwriter tops the main stage on a second day at the Finsbury Park event on Sunday, June 19.
Bob Dylan had previously been announced as the headliner for Saturday June 18.
Organiser Vince Power said: "Because of the popularity of the first day and the number of bands wanting to play, we have had to extend the festival to a second day.
"I am delighted to confirm Van Morrison to headline the second night and this is shaping to be a great celebration."
Other acts confirmed for the Irish festival include The Cranberries, who have recently reformed, Shane McGowan, Afro Celt Sound System, The Undertones, Imelda May and The Waterboys, among others.
Last year, Dylan and Van Morrison both headlined Vince Power's Hop Farm Festival in Kent.