Laya have a policy called Signify Plus, it's €490 for a young adult on that.
I only know about the Laya ones because we are with Laya.
EDITED:
The VHI plan you mentioned is Young Adult (18 - 20 yrs) -
€400.77 so that's cheaper than the Laya if the person is just turned 18.
There's a comparison below between the VHI one you mentioned and the Laya one.
Compare health insurance plans across a wide variety of providers in Ireland. You can also compare your current health insurance plan with other plans available.
Actually, remember to
check the Young Adult prices each year as they charge more as the person gets a year old.