A rather unusual little ditty.
My interpretation of it is as follows:
It begins with a parson or church minister who has a pair of people to marry or wed.
Bur before he can marry them, some hounds, or dogs, or perhaps even bad men, came into view, or came into the church, which resulted in the parson, throwing his cloak over his head and running always bidding them adieu, or good bye in French.
Mow the man who was supposed to be getting married, cried out or called out "tally ho!" Which indicated that the parson running away, was like a fox running away from the hounds.
The man who was supposed to get married to Nancy who he loved dearly, "all his fancy dwelt on Nancy" considered the parson a bad man to have run off before he finished marrying him, so called the hounds to go after him, by shouting or crying out,"tally ho!"
I hope that helped