You have to remember that the First Division winners will be a Premier Division club by the time the Setanta Cup comes around. A First Division club could have won the League Cup.
Also, there is a good chance that a First Division club will be representing the eircom League in both the Setanta Cup and the UEFA Cup if Longford win the FAI Cup. So, if Longford are relegated and win the FAI Cup, do people then think that they shouldn't be in the Setanta or UEFA Cup.
Remember, the First Division Champions only get a shot at playing in the Setanta. They still have to get by the Premier club. It's up to Derry to prove that they deserve it more than Harps or Cobh. If Harps or Cobh beat Derry, then they deserve to be in the Setanta. Same way as if Longford beat Cork, they deserve to be in the both cups next season.