This is the last of three automated Lightships, (officially referred to as lightfloats since they are automated), which were rotated between two lightfloat stations South Rock and Coningbeg while the third was a spare awaiting or undergoing refit.
The other two vessels were Kittiwake and Skua which have since been retired.
Gannet was last placed on the South Rock station; which is why the name of that station is painted on the vessel’s side. She has now been permanently withdrawn and has been replaced with a Superbuoy.
In the 1960s The Commissioners of Irish Lights had a fleet of 15 lightvessels. Each vessel had to be dry-docked, refitted and overhauled every two years so they were rotated between stations as refitting was due. This means a succession of different lightvessels would have been placed on each station.
You can learn more about the lightvessels on the Irish Lights website.