Irish rockstar Bono revealed that the new U2 album was extremely personal and difficult to do.

Bono, along with his bandmates, The Edge, Adam Clayton and Larry Mullen Jr., partnered with Apple to release their new "Songs of Innocence" to iTunes users for free. The said album is a compilation of tracks based on the band's past memories - some very painful to revisit.

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The musician told Rolling Stone that they wanted to make a personal album and try to put into lyrics and melody why they had wanted to be in a band in the first place, and the relationships they've had throughout their journey.

"The whole album is first journeys -- first journeys geographically, spiritually, sexually. And that's hard. But we went there," Bono said.

Bono pointed out a particular track in the album entitled "Iris" where the singer sang about his deceased mother, Iris Hewson, who passed away when the singer was only 14-years old.

He said his mother had passed away forty years ago at her own father's funeral.

"Rage always follows grief, and I had a lot of it, and I still have, but I channeled it into music and I still do. I have very few memories of my mother, and I put a few of them in a song called 'Iris'," he said.

U2 collaborated on several tracks with other musical greats such as  Ryan Tedder, Lykke Li, Danger Mouse and Paul Epworth. 

Bono credits his fellow artists for making the album better than what was imagined.

"Some of the music out there now that people call pop, it's not pop - it's just truly great. And we wanted to have the discipline of the Beatles or the Stones in the Sixties, when you had real songs. There's nowhere to hide in them: clear thoughts, clear melodies," he added.