Irish has things called initial mutations, which change the initial consonant of a word. mhairteoil is the form of the word after it has undergone the mutation called 'lenition', and mairteoil is the normal form. It's similar, in a way, to how English sometimes uses 'a' but uses 'an' at other times.
Another user (whose username I´ve forgotten) left this link in another discussion. It give pronunciations of Irish Gaelic words. Strangely, "mairteoil" isn´t listed, but "mair" is. I haven´t figured out whether DuoLingo follows the Munster or the Connacht pronunciation, but it sure doesn´t use the Ulster pronunciation. http://breis.focloir.ie/en/fuaim/mair