I'm so glad you asked. If the venue at which you are holding your fundraiser has an ASCAP license, then no, you probably don't need to get permission. (An ASCAP license covers public performances of songs by ASCAP-songwriters like Declan Patrick MacManus.) If the venue doesn't have a license, or their license doesn't include your type of event, you'll need to obtain an ASCAP license. (Songwriters who are not affiliated with ASCAP are usually affiliated with BMI.) Of course, if you're hosting a national event, you may want to get the okay from the songwriter to avoid any negative post-fundraiser blowback.
If you do anything other than play the song, you'll need more. For example, if you create a political video using the song, you'll need a sync license. If you use the recorded song as part of an ad campaign, you'll need other permissions, including one from the performers (under a principle known as right of publicity.) By the way, the fact that your fundraiser is nonprofit or for-profit won't make much difference in terms of your permissions. The Dear Rich staff could go on and on about these rules (we did write the book) but this Slate article says it more succinctly.
