You're right on the pronunciation and that is one of the reasons I was interested to find out if Dough and Dow were connected. My husband is English,so I was aware of the difference in pronunciation :-)
I have also seen the Dow variant with the e at the end. Henry Dow who came to America in 1637 signed his will as Henry Dowe. He had also used Dove at times.According to The Book of Dow, the final e was lost about 1725, but has been resumed. Several branches in CT had resumed the e for 3 generations at the time the author wrote the info in 1926.
Dow is not indigenous to Ireland or Wales, and Dows now in those countries are from families orignating in England or Scotland.