Under the Constitution of Ireland, a general election for the Seanad was required within 90 days of the dissolution of the 29th Dáil on 30 April 2007.

Polls closed on 24 July 2007, and the Taoiseach's nominees were announced by Bertie Ahern on 3 August 2007.

On 13 September 2007, Pat Moylan (FF) was proposed as Cathaoirleach by Donie Cassidy (FF) and seconded by Dan Boyle (GP).

The following table shows the composition by party when the 23rd Seanad first met on 13 September 2007.

This is a graphical comparison of party strengths in the 23rd Seanad in January 2011.