If I Forget Thee, O Jerusalem, Let My Right Hand Forget Her Skill

I found that the meaning of this prayer is to remember the calamities that Jerusalem has experienced.  The article specifically states "The meaning here is, that to sing in such circumstances would seem to imply that they had forgotten Jerusalem; that they were unmindful of its sorrows, and cared not that it was desolate. The remembrance of its calamities pressed hard upon them, and they could not do anything which would seem to imply that they had become unmindful of the sufferings that had come upon their nation".  

I underlined sing because this prayer is the chorus of one of Matisyahu's songs "Jerusalem".  I find it very ironic.