The relevant biblical passage is Hebrews, 7, 1-3 (King James version):

"For this Melchisedec, king of Salem, priest of the most high God, who met Abraham returning from the slaughter of the kings, and blessed him;

To whom also Abraham gave a tenth part of all; first being by interpretation King of righteousness, and after that also King of Salem, which is, King of peace;

Without father, without mother, without descent, having neither beginning of days, nor end of life; but made like unto the Son of God; abiding a priest continually."

Melchisedec turns up only in Irish forms of the song, so far as I know.