Welcome Jon !
When you say you are going to fish the Blackwater, I am presuming it`s the Munster (Cork) Blackwater. As there are 11 Blackwaters in Ireland, please let me know if I am assuming wrongly.
The Blackwater rises in Co. Kerry, and flows through Co. Cork entering the sea at Cappoquin Co. waterford, in total 105 miles long. If you know specifically what area or beats you intend to fish, let me know, and perhaps I can advise in more detail.
Water levels will dictate your fly size, but for this time of year, shrimp/hairwing patterns, usually trebles size 10-14 will cover most conditions.
My favourites for the Blackwater would be Gary Dog, Stoat`s tail, Silver Stoat, Munroe Killer, and Hairy Mary. Thunder and Lightening also works well.
I am not aware of any website that gives tying details, but I am sure there are some. If you need some fly-tying contacts, drop me a message and I will be delighted to give you some phone numbers. All good tackle shops there will stock these patterns, should you prefer to purchase them.
You have right to be excited, as this river can be very productive at this time of year.
One cavaet though, our Government, in all it`s wisdom, under the guise of conservation, has just this week introduced a law that any fish caught during the month of September, must be released. This rule has caused a lot of controversy and may be reversed, but as it stands, it is law.
You will be made most welcome on your visit, enjoy the Black stuff in Dublin, and tight lines in Cork.
If you need anything further, drop me a message, and I will be delighted to assist in any way I can.
Luck, is when preparation meets opportunity.