IIRC, either VLM or Denim has tendered for the Irish PSOs in the past. It is not that unusual. airbaltic tendered for a German PEO a while ago and was only narrowly defeated. I guess one hurdle to overcome for a foreign carrier is to convince the department in charge of the tender that not having the headquarters / maintenance etc. near the operation is no disadvantage.
As far as Aer Arann not having a suitable aircraft type available for the thinner domestic routes, a way out of this could be sub-contracting the PSO flights. Meridiana und Air One have done that for ages in Italy. The PSOs in Sicily were operated by German carrier Avanti Air on behalf of the local airlines.