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How do Goidelic and Brythonic languages differ within Celtic literature?

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An overview of the Irish language — Celtic literature Britannica editor Jeff Wallenfeldt provides an overview of the Irish language.

In the context of Celtic literature, the distinction between Goidelic and Brythonic languages is primarily linguistic, rooted in their development from an ancestral Indo-European form. While both are branches of the Insular Celtic languages, they are distinguished by their treatment of the ancestral sound kʷ.12

Linguistic Differences

Goidelic (Q-Celtic): This group, which includes Irish, Scottish Gaelic, and Manx, retained the q sound (which later developed into c).13
Brythonic (P-Celtic): This group, comprising Welsh, Breton, and Cornish, developed a p sound from the same ancestral source.13

Literary Context

These linguistic differences are reflected in the distinct literary traditions of the two groups. Irish literature, representing the Goidelic tradition, features early evidence in ogham inscriptions and a long history of professional bardic orders and filid. Conversely, the Brythonic tradition—most notably Welsh literature—possesses an unbroken tradition from the 6th century, characterized by heroic poetry and the development of intricate metrical systems like cynghanedd, as well as a rich prose tradition exemplified by the Mabinogion.34