Mishna Berura brings Elya Rabba who is matir a wedding the night of Lag B’omer b’shaas had’chak (such as when L”B falls on a Friday). I believe that the poskim today consider it a sha’as had’chak every year because there are not enough simcha halls for everyone to get married during the day.
Concerning haircuts, although it is intrinsically more lenient, sha’as had’chak does not apply to the same degree.
Regarding a haircut on motzaei Shabbos when L”B is on Sunday, it would seem pashut that since the heter for a haircut on Friday is l’kavod Shabbos, this would not apply to M”S, and it would have the din as any L”B eve. This sevara appears in Halichos Shlomo. It is of interest to note, however, that this sefer quotes RSZA as being matir haircuts on L”B eve even in an ordinary year, reasoning that it’s become somewhat of a yom tov (“They’re dancing in Miron, so you can take a haircut in Yerushalayim”).