Getting just over 50% of the company would have been useless to them. All they would gain is putting in place their own board and management. They wouldn't get access to Aer Lingus money unless they had something in the region of 75% from recolection from a media report on it. Ryanair themselves are quoted as saying they wanted at least 90% for it to work.
50.001% gives you control over board appointments and at AGMs but Minority shareholders do have rights in a Public Limited Company (PLC), some things cannot be voted through unless you have 75% of the vote of the shareholders in favour so having more than than effectively ensure certain things can be changed in Articles of Association etc.
The 90% issue is that if you hold 90% of the shares in a PLC then you can legally acquire the rest of the shares of dissenting shareholders at the offer price which the other shareholders accepted at, it can't be more and it can't be less.
The reason you would want to acquire the rest of the shares when you have 90% is that you can cancel the listing of a PLC on a stock exchange which is very expensive in terms of costs both in Legal, Audit and Statutory reporting with no benefit to the company as shares won't be traded .......£1-2 million a year is rough guesstimate cost. Private companies have less legal requirements.
Example is Glazer family acquired last 9.9% of shares in Manchester United Plc once they had got to 90% even though the shareholders didn't want to sell.......it then reverted to Manchester United Ltd.
It happens all the time with companies taken over as most shareholders sell once 50% reached as realise have little or no power after that.