It's mainly to do with Aer Lingus fervently pursuing the 'low-cost' business model and doing whatever is in it's best interests with little dialogue with Oneworld partners.
It's felt that the transformation to economy only on European routes and the paid-bar (i.e. a genuine low-cost model) doesn't 'fit' well with Oneworld any more.
Also EI has tread on IBs toes with rapid expansion throughout Spain etc and continued expansion throughout Europe is seen as 'stealing' connecting pax through MAD and LHR.
Couple that with the effective shutting down of TAB (EIs frequent flyer program) and there seems little incentive to either side for participation any longer.