"Bu toil leam an t-uinnean."
Translation:I would like the onion.
Is n before some words in English, like in an apple, redundant (as if a a apple)?
It’s not redundant in Gaelic either, it’s just a form that the definite article takes before masculine words starting in a vowel: an t-athair the father, an t-ubhal the apple, an t-ainm the name.
(The historical reason for this is that the masculine nominative article was *sindos in Proto-Celtic, which changed to *indoh somewhere in early Primitive Irish, then lost the unstressed o becoming *indh → int before vowels, eg. Old Irish int athair the father, int ech the horse, modern an t-athair, an t-each, but before consonants the -h was lost earlier and d got assimilated to n – *indo(h) became in, eg. in fer the man, in macc the son, modern am fear, am mac)