The Blackwater is easily accessible by road from Cork in less than 1 hour, from Dublin in about 2.5 hours, from Rosslare in about 2.5 hours, and from Shannon in under 1.5 hours. You aim to ultimately pick up the N72, which follows quite closely the general course of the Blackwater, especially west of Fermoy. (The R666 may be more convenient than the N72 if your destination is east of Fermoy and on the north bank of the Blackwater.)
Note that a car will likely be required to travel between your fishing beat(s) and your accommodation.
The Republic of Ireland has three international airports: Cork Dublin and Shannon
Between them, they serve many routes to and from European countries and the USA and many carriers, including:
and several others.
Cork airport is especially convenient due to its close proximity to the Blackwater. Car hire can be arranged at each of these airports.
There are several direct sailings between Ireland and the UK and between Ireland and France using terminals at Ringaskiddy, Co. Cork; Dublin Port; DunLaoghaire Port, Co. Dublin; and Rosslare. Sailings also exist between UK and several parts of Europe for those planning to make the journey in two stages. See the following websites for details:
P&O Irish Sea Ferries
The following site may be helpful in comparing prices etc.