I take it you are coming from overseas.
I would imagine it could be difficult for you to fish local club comps as you would not be a member. Difficult but not impossible, there are always way around these things!
The Cobh International takes place early each September. This is a charter event. Check the following sites. There’s not a huge amount of info but you could contact the Geary’s as they are very much involved with the event.
http://www.seafishing.irl.com/
http://www.cobhseaanglingclub.org/
You will find details elsewhere on this site of charters operating out of Cork harbour. Most if not all of these boats would be involved in the championships.
If you can organise a small boat then Cork Small Boats Festival takes place in late September.