Ifin, also written Pin, was devised to carry the sound p, and its existence points to something specific about the language the original ogham alphabet was built for. Primitive Irish, the language the twenty core letters were cut to spell, had no native p sound: it had been lost from Proto-Celtic by sound change before the alphabet was designed, which is why no letter in Aicme Beithe, hUatha, Muine or Ailme carries it. By the time Irish had reacquired a p sound, chiefly through borrowed words from Latin and later languages, the working alphabet needed a fifth group of letters to hold it, and Ifin is the answer.
The Auraicept na n-Eces glosses the letter with spinan no ispin, gooseberry or thorn, continuing the plant naming convention used across most of the forfeda. The gap it fills is not a minor refinement but a structural absence built into the alphabet from its first design, which makes Ifin a useful marker for how much the sound system of Irish had shifted between the fourth century stone carving tradition and the manuscript scholarship that added this letter centuries later. The dual gloss, gooseberry or thorn, is itself a sign of the uncertainty running through the forfeda tradition: even the tract copies disagree on which plant word the letter is meant to carry.
Sources Auraicept na n-Eces: The Scholars' PrimerGeorge Calder (ed.) with A Guide to OgamDamian McManus