You should probably first define what is North Italy and what is South Italy. I feel that Italy, just like France, is a country too diverse for such a simplistic dichotomic partition.
Personally I perceive at least 4 major ethno-cultural macroregions among ethnic Italians (Aosta Valley and Südtirol are thus excused), which are based primarily on genetics and culture:
- Northern Italy: Piedmont, Lombardy, Veneto, Trentino, Friuli Venezia-Giulia
- Central Italy: Liguria*, Emilia-Romagna, Tuscany, Marche, Umbria and Latium*
- Southern Italy: Abruzzo, Molise, Campania, Apulia, Basilicata, Calabria, Sicily
- Western Italy: Sardinia
*Now Liguria and Latium are the tricky ones: Liguria genetically is transitional between Central Italians and North Italians (with the western half being closer to the North Italian cluster and the eastern half basically merging with Tuscans/Romagnols), however its culture, climate, architecture, lifestyle and overall "feel" resemble more that of Corsica and Tuscany in my opinion than anything north of the Po.
Likewise, Latium is genetically transitional between Central and Southern Italians; as for the culture though it's very much intertwined with Central Italy I reckon, except maybe the southernmost parts. So this is why I grouped these two regions with Central Italy.
Moving on. About the North Italians and the Celtic look nonsense, please define the Celtic look: is Celtic look that of the irish Conan O'Brien or perhaps that of the breton Benoît Hamon? People need to realize that Celtic is a language family, not an ethnicity. Just like the label Slavic, you can range from obvious balkanites like the Bulgarians/Macedonians to Russian speaking "pseudo-Balts" living in Latvia and Northwest Russia.
Either way, North Italians look phenotypically halfway between Central Italians and the Swiss while Southern Italians probably more like halfway between Central Italians and Cretans.