You're wrong. The first part of Sean and Seamus only look similar because English leaves out the length marks of Irish Gaelic orthography; the Irish spelling of Sean is Seán, and the Irish Pronunciation is [ʃaːn] or [ʃɑːn]; Seamus, on the other hand, is Séamas, pronounced [ˈʃeːməs] (compare the Wikipedia article on Irish Orthography ⚠️ ↗). The e in Seán is not really a vowel sign, it merely shows that the s is palatalized even though it is followed by a (long) back vowel; conversely, the first a in Séamas signifies that the m is not palatalized, despite the preceding front vowel.

All the relevant English pronunciation dictionaries — the CEPD ⚠️ ↗ (as well as its predecessor, Daniel Jones's legendary EPD), the ODP ⚠️ ↗ and, best of all, J. C. Wells ⚠️ ↗'s LPD ⚠️ ↗ — agree that, in English, Sean is pronounced [ʃɔːn] (with an American variant [ʃɑn]), while Seamus is pronounced [ˈʃeɪməs].



Surf safe;)
NFiH