Judging by the fact that there is only one other member in Ireland apart from the Kinvara Boys I'm guessing this may produce a bit of a poor response. I thought I'd posted this a few weeks ago but it's not here so who knows where I posted it!!
There is a company called dinghy-bitz (www.dinghy-bitz.co.uk) they can do heron spars:
http://www.dinghy-bitz.co.uk/acatalog/Heron.html
They don't seem quote if they charge extra to deliver to NI, they normally do free delivery but not on masts.
They look like they supply SuperSpars stuff. McCreeady Sailboats (http://www.mccreadysailboats.co.uk/ ) are the NI Dealer for SuperSpars. They are based in Holywood which I guess is handy for you. Would be worth checking all the dimensions in the rule book before ordering as I don't suppose you can just walk in and say 'I want a heron mast' to these guys...
You can get two part masts (not sure who from) which may actually make delivery cheaper because its the length that makes the delivery so crazy.
My Boom came from Butler Boats, although Butler actually ordered it from Custom Dinghy Spars (01332-811131) they trade online as
http://www.evolutionsailing.com/ - they don't mention Heron's on their website... Don't want to take the business away from Dave Butler as clearl;y he is important to the Heron World, but may be easier with your awkward

shipping needs to contact them direct.
The Kinvara boys made their own from Southern Yellow Pine... if you are feeling adventurous! (They look beautiful)