I don't actually know the proper pronunciation and I sort of like not knowing. I actually think it MIGHT be pronounced UUL with the r silent the same sound as Spool/Fool/Tool but this YouTube vid says it's like Ooul-ar
http://norse-mythology.org/ullr/
https://www.youtube.com/watch?v=56ngMG_ljFs
that's some weird pronunciation lol but the god himself is fucking bad ass.