There were many Celtic loan words in their vocabulary, and they made a round leather coracle or bull-boat to pole along in the Missouri R.
All who saw the Mandan were aware that they represented a Euro-Amerind [mestizo] people of some kind, probably mixed at a not-too-remote date, since even Lewis & Clark said that all hair and eye colors seen in Virginia whites could be seen, by exception, in the [two large] Mandan villages.
Runaway Norsemen from the Greenland and/or Vinland colonies, see Kensington Runestone, have also been suggested; the two are not mutually exclusive, but archaeological remains suggest the Mandan had been driven down the Ohio and up the Miss/Mo River systems, until at length they reached an area where their superior agricultural and living technology gave them an advantage over other tribes' numbers, enough to survive.
The Mandan had a vassal (slave) sub-tribe, the Hidatsa, who dwelt near and around them on the prairie and seem to have picked up some of their ways at 2d hand. IIRC the language of the Hidatsa was Siouxan, however, while that of the Mandan was a mix of Celtic with Shawneean or Ohio-Valley tribes' languages.
The Mandan mostly perished in a smallpox epidemic about 1839-40 after their chief had turned down a US offer to vaccinate the whole tribe. Survivors took refuge and mingled with the Sioux and other tribes of Indians before any adequate ethnological study could be done. See accounts of them in Catlin and in Lewis & Clark.