The Dubliner (University Ave @ Vandalia) has been my "go to" for Guinness for the past three years. Just recently it seems to have wobbled a bit (but that may be the hot weather). It also has one of the nicest, longest, bars and bar backs in town.
I was recently at O Donovan's and theirs was not at all bad, perhaps aided by an english bartender.
Mayslack's also do 20oz Guinness which (once you run a through pints through the pipes as I don't think they have many Guinness drinkers) is not horrible, especially when $2.50 a go on Monday Happy Hour. .
Speaking for downtown and I this retiree is getting a little dated I knew there was quite a distinction between two restaurants downtown in the Local and the Newsroom. The Local was a topnotch pour but the Newsroom was consistently stale compared to the Local. Being adjacent it was easy to compare to confirm that. I suspect it probably has more to do with volume and how fast an open keg gets dispensed. I also thought the Local had bartenders trained to properly double dispense a Guinness. Most of the Irish claimed pubs in the Twin Cities I've been to have poured a decent Guinness, IMO. I think so because most of them know it is an important aspect of their business. I haven't been to Ireland to give you a comparison. I have been to England and Wales and ordered Guinness at most of the 20 or so pubs I sampled.