Q. What is the earliest time I can cut my hair on Lag Baomer, can it be done erev Lag Baomer after already reciting Maariv, following the Plag Hamincha?

A. Remah (O.C. 493,2) rules that one should wait until the morning of Lag Baomer day for having a hair cut. Mishna Berura  (ibid. 10) explains that it refers to after the netz. However, he points out (ibid. 11) that there are Poiskim who permit a haircut on the night before, from Shaar Hatziun (ibid. 12) it seems that he does not agree with them.
Nitey Gavriel (Pesach 3, 49 n. 21) explains that the above depends on the minhag of  reciting or not reciting Tachanun during Mincha erev Lag Baomer.

Horav Shlomo Miller’s Shlit”a opinion is that even for those who permit hair-cutting on the night before, they definitely not include the time after the Plag before nightfall, despite having prayed Maariv after the Plag.

Rabbi A. Bartfeld as advised by Horav Shlomo Miller Shlit”a