Ground-based will have the problem of line of sight/range. I guess in NZ this can be improved by placing them on their plentifull supply of high mountains (not available in Ireland...), but this would also cause many blind spots (because of said mountains); and you'd have the added costs of building the road infrasture and support for this isolated sites. Civilian airlines tend to follow set flightpaths, with makes ATC a lot easier, but if you want proper EW and GCI, you need a radar way up, to improve LOS and eliminate ground blind spots (hills, mountains, canyons, etc.