I found myself wondering the same thing and was reaching the same bizarre conclusions as you (i.e., "cúl báire" = "goal goal"??) -- I even came into the discussion tab specifically to see if someone had already answered this. Since there wasn't an answer already here, I looked around in Teanglann a little more and found that the entry for cúl had extra definitions for the word that give this term a clearer context. According to Teanglann, the primary definition of cúl is "back," with a secondary definition of "support," so therefore using cúl to refer to a person could imply that they are a "backer" or "supporter" given the correct context. Thus, a cúl báire would be a "backer of a goal" or a "supporter of a goal" (since the g.s. of báire is still báire). As my best guess, I think that's where the meaning of "goalkeeper" comes from.