I haven't had their Irish Stew, but everything else I have had at Fibber McGees has been great! I went there on a recommendation from an Irish expat living here who said it was the most authentic Irish Pub she had been to in Arizona.
Fibbers is located on the SE corner of Dobson and Elliot
I've actually had this at Rula Bula before, though I've never had it anywhere else so I can't really compare it to anything. From what I remember it's a big, shallow bowl with a lot of root vegetables and big, tender chunks of lamb. The lamb was not particularly fatty, which I appreciated. They didn't oversalt it, and it's not particularly seasoned but still made for a good hearty meal. Definitely a lot of food for the money.
Depending on the night of the week and what you're looking for, you might want to check with Rula Bula (if you go there) to see if they're having live music - sometimes it's obnoxiously loud. It also gets extremely crowded with the carbomb-loving ASU crew on the weekends.
It's on the menu at Rula Bula in Tempe. I have never tried this particular dish, so I cannot attest to the quality of Rula Bula's preparation of it. I have found Rula Bula to be somewhat erratic. I like the decor and the beer; the food and service have been inconsistent during my visits.
Most other Irish pubs in the PHX area, such as Seamus McCaffrey's and the Skeptical Chymist, appear to offer beef stew instead of lamb stew.