And there is always the possibility of transcriptions being phonetic.
Longford GAA website give this:
Club Name: Longford; As Gaeilge: An Longfort - Shane's first thought...
However goireland.ie has it like this:(I think its originally from McLysaghts Surnames of Ireland)
.....O Fearghaill (O'Farrell or O'Ferrall) sept was of Annaly in Col Longford. The chief to the sept, known as Lord of Annaly, resided at Longphuirt Ui Fhearghaill (I.e. O'Farrell's fortress), hence the name of the town and county.......
So maybe it changed when the Teanga was standardised.....
PM