Built in 1817, this light served Galway harbour for 160 years until replaced by Leverets and left to rot. There have thankfully been recent efforts to restore it to its former glory, (it appears to be freshly painted) though it now shares the ile de mouton with a sewage treatment plant.
To reach it, you need to go to Claddagh. Heading west out of the city centre from Spanish Arch take the turn to the left on the far side of the river. When you hit Grattan Road, you should see the light. There is onroad pay and display parking. The causeway is actually much longer than you think and stay on the Galway city side of it if you want to avoid a drenching!