My understanding of why Aer Lingus do not want to use the CBP facility at this time is as follows:
The current setup which has CBP installed in SNN but not yet in DUB would require, with EI133 DUB/BOS for example that all passengers and baggage transiting SNN be offloaded. The baggage would then be xrayed and made available to CBP staff should they wish to inspect it. Once all passengers have cleared CBP facility then EI will be permitted to reload the baggage.
With just 1 hour turnaround time and the possibility of 150+ transit passengers it simply does not make logistical sence for EI to proceed at this time. Once CBP is established in DUB, these same 133 pax will have their baggage processed at DUB in a manner compatible with CBP requirements and therefore avoid the requirement to offload them in SNN.