Hi Sunster,
No it doesn't effect the durability. It has been acceptable in the past, but I think it is going to be phased out soon from the building regulations. It is just that the cavity wall construction is a better system. That's not to say the life span of the house will be any different for the Lusk houses- just a better performance with regard to insulation and being draught/weather proof. It's just that construction methods are evolving all the time.
Lots of people are buying older houses from 1950's and before which wouldn't have anywhere the same construction standards as now and they're still performing satisfactorarily,probably just costing more to heat. So I wouldn't let it worry you unduly.
I'm very happy with 4 bed detached. I had a deposit on the 3 bed with car port but changed over to the 4 bed and have't regretted it. They were €320K then. I put a deposit down in April '04 and had moved in by the following September. Best of luck!