What the League/FAI should do is say to Shels and Sligo that the minute(s) when the Sligo player should have been off should be replayed with the scoreline as was at the time of sending off.
In that way Shels will probably not bother playing a couple of minutes and say Good Luck to Sligo ( I think Sligo had scored before the player got two yellows ).
It would be a fair decision and would mean that the cup can go on.