Synonyms for irrelevant

Most common

1. Inapplicable — not relevant, as, Not usefully related, not associated in any way that is important or useful to the context…

2. Unrelated — not relevant, as, Not at all related, totally unrelated, wholly unconnected, completely inapplicable

Other

beside the point, ill-timed, expired, irrelative, otiose

adj. 1
Not relevant, as: Not at all related: totally unrelated, wholly unconnected, completely inapplicable.
adj. 2
Not relevant, as: Not usefully related: not associated in any way that is important or useful to the context being discussed: not materially applicable; having connections not applicable in the present situation.

Irrelevant, Impertinent or Inappropriate?

Impertinent and irrelevant matters can not claim consideration in a certain connection; inappropriate matters could not properly be considered.

J. C. Fernald, English Synonyms and Antonyms (1914)

unimportant This detail is unimportant. You can safely omit it.
immaterial Age is immaterial, unless you're a bottle of wine.
tangential The liquid flow submits the container to what we call tangential stress.
inapplicable This article of the constitution was declared inapplicable.
random Tom opened the book at random and started reading.
unrelated Most of my sentences are unrelated to anything here.

Use in printed books, per decade from 1850 to 2019.

4 letters: moot¹
5 letters: alien³, unfit², inapt, unapt
6 letters: random², remote², otiose³
7 letters: foreign², expired²
9 letters: unrelated⁴, desultory⁴
10 letters: unsuitable⁴, incidental⁴, stochastic³, orthogonal⁴, immaterial⁵, extraneous⁴, tangential³, inapposite, irrelative
11 letters: unimportant⁴, unconnected⁴, impertinent⁴
12 letters: inapplicable⁵, uncorrelated, unassociated

The small number after each word is its number of syllables (alien³ = 3 syllables).