For what it's worth, and the rules may be different now, but as an Irish citizen I joined the RN as a pilot in 1988. It took a couple of years after joining before they made me get naturalised as a UK citizen. I then held dual nationality (Ireland/UK), have not long left the RN, and now have reverted back to just using the Irish passport. I had no snags begin accepted into the RN, except for a very long time to get security cleared before entry, but that's probably due to my Grandad being in the IRA in the early years…..