All the records for the William you cited in your original post relate to the son of Paul and Catherine (Kate in 1901). If you are sure that this is "your" William, then the fact that he isn't with his family in 1911 is not unusual, and you should, I suggest, ignore it. The other 1911 William Keogh may have been one of the others who served in the Royal Irish or indeed in another regiment so the first thing to decide is whether you are content that William, son of Paul and Catherine , born in Kings County in 1899/1900 is yours.
maxD