Source: Larry Bristol (firstname.lastname@example.org), r.c.b., 5/11/96
I do not think you HAVE to use N2 to achieve the creamy head, but it would surely help. Otherwise, the longer the beer can be allowed to rest under CO2 at cool (serving) temperatures, the better the head. I keg and use forced carbonation. I will let the stout sit at serving temperature and under serving pressure for a minimum of 14 days before serving; it's better after 30 days. It seems to work!
The first (and perhaps most interesting) aspect of Guinness is that there appears to be more than one recipe! The stout served in Ireland is different from that sold in England and also from that exported to the US. I refer to these as the "Irish Stout", an "English Stout", and an "Export Stout". The priniple difference seems to have to do with the amount of sour mash flavor included in the brew. There could easily be other differences as well.
So when I brew my "Guinness-a-like", I also need to decide which one of these targets I am hoping to hit. I start with a basic stout recipe (see below) that makes what I call the "English" version; it has NO sour mash taste. Or start with whatever recipe you think comes closest and adjust from there. If I decide to make the "Export" or "Irish" version, I will sour the brew (after fermentation), with the "Iish" being the most sour.
As I understand, Guinness actually allows part of the mash to get "infected" with a lacto-baccilli (why can I not think how to spell this morning?). I did not want to fool around with that sort of thing (tough to control, lots of extra work, etc.), so I sour the beer by adding carefully controlled amounts of lactic acid after fermentation is complete. I add it to the keg as I rack from the secondary fermenter; if I were bottling, I would add it along with the priming sugar.
How much lactic acid? You'll have to be the judge of that for yourself as you
decide how "Irish" versus how "English" you want your stout.