Hello Diane-

I don't know how much this is going to help you as I can't specifically address your questions about the Mulhern-Sullivan boarding house or ED 1149 in Cicero. Perhaps, however, I can bring you one step closer to a resolution.

I can tell you that I am quite certain that the location you are looking for is located in the present day Village of Oak Park (not Cicero) located just to the north of the Town of Cicero. In 1900 Oak Park was part of Cicero Township (it broke away in 1901-02).

There are a couple of factors I base this on...

-Cicero doesn't have addresses anywhere which would be 217-218. Oak Park would have such addresses.

-I can't tell you exactly where your ED 1149 is located, but it is clear to me it is in Oak Park as the following "famous" Oak Parkers are shown as living in the Town or Township of Cicero on the 1900 census when all of their homes are now landmarks within the corporate limits of the Village of Oak Park. Indeed, all subsequent censuses (1910, 1920...) show their homes in Oak Park:

Frank Lloyd Wright (architect) ED 1146
Richard Sears (Sears Roebuck) ED 1148
Ernest Hemingway (writer) ED 1146

So, you might want to focus your search on the Village of Oak Park. Might I suggest you contact the Oak Park Historical Society. I don't know how much they can help you as far as the specific boarding house you are looking for, but I am certain that they can easily supply you with the location of your 1900 ED 1149.
Heres their website:

http://www.oprf.com/OPRFHIST/

I am quite familiar with both Cicero and Oak Park, so if you can give me anything more to go on, perhaps I can help a bit. I think once you obtain the specific boundaries of ED 1149, perhaps we can narrow down the location of the 1900 boarding house you are looking for.

Wish I could have been more helpful. Please feel free to contact me here or via email if I can help in any way.
Paul