Fellatio, you should know the answer to this question: If the Irish allowed U-boats to be stationed in Ireland, how big a threat would this pose to Britain and the Royal Navy and how effectively would the Royal Navy be able to deal with this new problem?
Depends entirely on how it was done, O Sudanases one.
Obviously, doing it publicly would have been a complete no-no to the vast bulk of the Irish population and Establishment and invited bombing by the RAF and/or invasion from NI.
What was most likely - although unlikely - was the use of the wild and craggy west coast of Ireland as a refuelling and re-arming point for U-boats; perhaps with a support vessel or else reliant on depots created ashore, perhaps created by other U-boats depositing fuel, certain spares and torpedoes: certainly no dockyard facilities as such. (AFAIK, the Germans could well have used a similar tactic to rest and resupply in between patrols, albeit without the knowledge of the Irish Govt.)
This would have greatly assisted turnaround time for subs in between patrols, as they would not have had to go back to France, Norway or Germany; especially as they would be proximate to the concentrated shipping routes that terminated in Belfast and Liverpool.