Purely as a shareholder, Ryanair have made a poor return on their investment.
As one of Aer Lingus' major rivals, Ryanair by virtue of their large minority shareholding will have been able to significantly influence the future of Aer Lingus, if only by virtue that they had an effective power of veto on any merger / takeover involving EI. Power to significantly events around a major rival has a value to which it is difficult to ascribe a price