I think they had to register last year or they were confined to the national event. Don't think that's the case this year.
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I was offering to make "national" priority, which should be much cheaper (lets say 500 euro), when the drivers will be allowed to start in front of the field, but without chance for prize money. But it was refused, AFAIK by FIA...
It seems Martin McCormack has already registered:
https://www.fiaerc.com/driver/martin-mccormack/
I guess at least some Irishmen will do also Ypres and in that case it makes sense to register.