There is no mh in Go raibh maith agat - the "v" sound comes from the bh in raibh, and that is only in Munster Irish! In both Ulster Irish and in Connacht Irish, there is no "v" sound in raibh, or in a broad mh, and even in Munster Irish, a broad mh doesn't always have a "v" sound
https://www.teanglann.ie/ga/fuaim/oíche_mhaith (mh is "v" only in Munster)
https://www.teanglann.ie/ga/fuaim/tráth_dá_raibh (bh is "v" only in Munster)
https://www.teanglann.ie/ga/fuaim/mí_na_Samhna (no "v" sound" even in Munster)
https://www.teanglann.ie/ga/fuaim/Mheiriceá (slender mh - "v" in all dialects).