Thought this was an interesting bit from the site Wellfan1984 linked to;
It's a different rule for the Cup winner being a Champions League entrant and a Europa League entrant;
-if the Cup Winner has qualified for the Champions League, it's the runner up that qualifies (unless they have already qualified through their league position),
-if the Cup Winner has qualified for the Europa League, the runner up does NOT enter the Europa League, the final place goes to the 5th place finisher.
So, from this, if Celtic beat Dun Utd in the Cup Final then a 5th place finish will get us into Europe. Also the same if Dun Utd beat Celtic. If Celtic beat Raith in the final, Rovers would qualify at the expense of the 5th placed team. If Dun Utd beat Ross County then we are in.
I imagine then the reasons that Dunfermline, Gretna, QOS, and Falirk have all made it in to Europe as Cup runners up recently, was due to the winning side having qualified for the Champions League. (Celtic, Hearts, Rangers X 2).
EDIT:
Noticed this post at the bottom of their thread;
The Scottish Cup winner will qualify for the UEFA Europa League Play Off Round.
The remaining teams are Celtic, Dundee United, Raith Rovers and Ross County.
If Celtic win their Semi Final against Ross County, the winner of the second Semi Final between Raith Rovers and Dundee United will be guaranteed to qualify for the Europa League providing Celtic qualify for the Champions League. If Dundee United win the Semi Final, 5th place in the SPL will enter the UEFA Europa League 2nd Qualifying Round providing Dundee United have qualified for Europe through the league, as it stands both Motherwell and Dundee United will each enter one round later - Motherwell QR3, Dundee United QR4.
If Ross County win their Semi Final, 5th place in the SPL will only qualify for the Europa League if Dundee United win the Scottish Cup and do NOT finish runner-up in the SPL. If Dundee United win the Scottish Cup and finish runner up in the SPL, Ross County will enter into QR2, with the SPL 4th placed team entering QR3 and Celtic entering QR4. If both Ross County and Raith Rovers win their Semi Finals, the Scottish Cup Winner will enter QR4.