About this phrase

Do is the possessive adjective, second person, singular, meaning your. The terminal vowel sound is lost here through coalescence with the following one and this phenomenon is indicated by an apostrophe in the official standard orthography.

Uimhir is a feminine noun that means number.

Teileafón is a masculine noun that means telephone. It is in the genitive case here because it is the second noun in a two-noun cluster.

Thus, d'uimhir theileafóin: your phone number.

Notes

Although the word teileafón is perfectly valid, guthán and fón have much greater currency in everyday usage. Following this:

• D'uimhir ghutháin: your phone number.
• D'uimhir fhóin: your phone number.
  • Well, not quite…
  • D'uimhir fhóin is technically correct according to the official written standard, so there's no reason not to use it in formal writing.
  • But it's one of those forms that are just too correct to use in naturalistic speech.
  • Use d'uimhir fóin in spoken Irish.
  • Compare with real-world usage of it's I versus it's me as answers to the question who's there? in English.

With regard to the mobile phone, fón póca and guthán póca (literally, 'pocket phone') are the de facto standard rather than the bookish teileafón soghluaiste and its variations.