An 1899 shows Dispensary Lane at a slightly different location to 1884, and between 95 and 96 Lwr Dorset, but it's possible the numbers altered over time. It looks like it's possible that it was part of the unlabelled street leading to Dorset Ave.
There were a number of Dispensary Lanes around Dublin, and I think they could have been named after streets where the local dispensary was located at one time.
Shane