If Ireland did that they would jumped on. Refueling u-boats would impossible to keep hidden. As for sightings strictly speaking they should not have been passed on but then again seeing as the very same u-boats were sinking ships bringing much needed supplies to Ireland not to do so would be frankly crackers.
Unless the RN picketed Ireland or the RAF patrolled overhead (both of which is more burden for the UK at the worse time), I'd say that you could get away with supplying U boats along the west coast out of the smaller ports (ie don't use the Treaty ports or Galway/Limerick maybe Tralee) with the level of OTL censorship you could reduce the chances of the information leaking from the Irish side.
You're right about the consequences for Ireland losing the supplies though if Ireland is a) nuts enough to go fascist they may not agree and b) if Ireland is Fascist leaning/hostile how willing would the UK be in regards to supplies?