Brenna Shanahan:

Brenna: /ˈbrɛnə/ or /ˈbrænə/ (depending on dialect; the first is more common in American English, the second more common in some British dialects). This reflects the pronunciation with the emphasis on the first syllable, the short "e" sound in "Bren," and the schwa sound at the end.

Shanahan: /ʃəˈneɪən/ or /ʃæˈneɪən/ (again, dialectal variation is possible, particularly regarding the first syllable vowel). This captures the "sh" sound, the schwa sound in the second syllable, the long "a" sound in "nay," and the final "n" sound.

Variations might include:

• Brenna: BREN-uh, BREN-ah (simpler, more common approximations)
• Shanahan: SHAN-uh-han, SHAN-ay-han (simpler approximations, emphasizing syllable breaks)

The phonetic spellings provided above use the International Phonetic Alphabet (IPA) where applicable, offering a precise representation of pronunciation. The alternative spellings offer a more readily understandable, albeit less precise, representation for those unfamiliar with IPA. The choice between these depends on the intended audience and level of phonetic detail required.