As for Baron Field Marshall Sir Edmund Ironside you can check with Burke's Peerage. The entry reads in part as follows: "The family of Ironside takes its name from a strip of land named Ironside, situated in Buchan a few miles north of Brucklay. The origin of the name is a matter of conjecture. It may either be from the fact that Edgar the Athling, grandson of Edmund Ironside settled there in the time of Malcolm Canmore, or it may come from some Scandanavian invader with the surname Ironside. In the 14th cantury the Ironsides were closely settled in the area of New Deer, but later are to be found farming all over Buchan. No exact details are available before 1696, though tradition has it that William Ironside settled in Ironside about 1400.
The Baron's ancestors are listed as follows:
John Ironside, given in the poll book of Aberdeenshire of 1696 as farming in Rothy in the Parish of Fyvie, b. 1636, m. Agnes Kennert.
He in turn was followed by John, Francis, William, all farmers, and then William, a Medical Doctor, Surgeon General in the Horse Artillery who was the Baron's father.
As for the MacKay connection I can shed no light.
I trace my ancestors back to farmers in the New Deer area.