Horgy, was in the same position as you. Am with the same said carrier that you mention, for arguments sake, lets call them Ryanair. Get the IAA Licence and upgarde it to ATPL. Job done, you have the Licence required for BA DEP Long Haul. IF you are then lucky enough to jump through the hoops at BA and get an offer, I am reliably informed that BA would indeed cover the cost of transferring it back to a CAA Licence. The cost of the initial transfer to the IAA costs around £200. The cost for a SIM, a TRE and hire a Friend for the day I don't have a clue but we must be talking £1k+.
Having been in your position, it's a no brainer - get the Irish Licence.