Circumflex In Welsh :

The circumflex is known as hirnod "long sign" or acen grom "crooked accent", but more usually and colloquially as to bach "little roof".

It lengthens a stressed vowel (a, e, i, o, u, w, y), and is used particularly to differentiate between homographs; e.g. tan and tân, ffon and ffôn, gem and gêm, cyn and cŷn, or gwn and gŵn.

However the circumflex is only required on elongated vowels if the same word exists without the circumflex - "nos" (night), for example, has an elongated "o" sound but a circumflex is not required as the same word with a shortened "o" doesn't exist.

the circumflex,

due to its function as a disambiguating lengthening sign, is used in polysyllabic words with word-final long vowels. The circumflex thus indicates the stressed syllable (which would normally be on the penultimate syllable), since in Welsh, non-stressed vowels may not normally be long.

This happens notably where the singular ends in an a, to, e.g. singular camera, drama, opera, sinema → plural camerâu, dramâu, operâu, sinemâu; however, it also occurs in singular nominal forms, e.g. arwyddocâd; in verbal forms, e.g. deffrônt, cryffânt; etc.

(Wikipedia)

Yes, the circumflex exists in the Welsh spelling:

Circumflex In Welsh :

The circumflex is known as hirnod "long sign" or acen grom "crooked accent", but more usually and colloquially as to bach "little roof".

It lengthens a stressed vowel (a, e, i, o, u, w, y), and is used particularly to differentiate between homographs; e.g. tan and tân, ffon and ffôn, gem and gêm, cyn and cŷn, or gwn and gŵn.

However the circumflex is only required on elongated vowels if the same word exists without the circumflex - "nos" (night), for example, has an elongated "o" sound but a circumflex is not required as the same word with a shortened "o" doesn't exist.

the circumflex,

due to its function as a disambiguating lengthening sign, is used in polysyllabic words with word-final long vowels. The circumflex thus indicates the stressed syllable (which would normally be on the penultimate syllable), since in Welsh, non-stressed vowels may not normally be long.

This happens notably where the singular ends in an a, to, e.g. singular camera, drama, opera, sinema → plural camerâu, dramâu, operâu, sinemâu; however, it also occurs in singular nominal forms, e.g. arwyddocâd; in verbal forms, e.g. deffrônt, cryffânt; etc.

(Wikipedia)