Denis O'Brien is a shrewd businessman and wouldn't be paying out that kind of money because of some fit of pique.
There are three reasons he may have made this investment
- Because he anticipates a higher Ryanair bid and thinks he can make a profit fro this
- Because he sees Aer Lingus as a good medium-term investment
- Because he thinks that the companies share price doesn't reflect its the difference between its assets and liabilities and there is money to be made by breaking it up