Q. Rashi writes in Taanis and Pesachim that there is a demon (sheid) called Igras and people don’t go out on Tuesday and Friday nights to avoid being harmed by this demon.
Question: How can a demon have dominion on Shabbos when it says in Zohar that all evil forces are nullified on Shabbos and have no dominion, and we don’t even daven for shemira on Shabbos (shomer amo Yisroel) in the blessings of kriasShema, because the merit of keeping Shabbos is itself a shemira? So how it can be that this demon should have dominion on Friday night?

A. Kovetz Shitas Kamoei (Pesachim 112b) quotes Meforshim on the sidur that we recite Hapores Sukas Sholom as a tefila on Shabbos for protection from sheidim, when we cannot use a torch to illuminate the darkness outside. Perisha (O.H. 267: 3) explains that although Shabbos provides shemira, however, in dark and dangerous places on the out of town fields, where their Botei Knesiyos where to be found, added protection and tefilos were necessary.
Horav Shlomo Miller’s Shlit’a opinion is that indeed keeping Shabbos is a great shemira, and the Yom Kodosh watches and cares over the ones that care and keep it properly and with the right intention. However, if one does not do so properly, one may need the added tefilos.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a