Yes, I know. Séamas (Irish) and Seumas (Scottish) are both pronounced Shame-us. However, when addressing someone in Gaelic (either variety), be it speaking or writing a letter, one uses the vocative case which changes the spelling and the pronunciation. The first letter is subject to lenition, which involves adding an 'h' to the initial consonant. This rule only applies to the letters b, c, d, f, g, m, p, s, and t. This changes the sound as well. The name is then put in the possessive, which in the case of Séamas/Seumas means placing a "i" before the s. When the letter "s" is preceded or followed by a slender vowel (in this case "i") this creates a "sh" sound.

So, Séamas becomes Shéamais and Seumas becomes Sheumais. (Both are pronounced HAME-ISH when using this form.) These are forms are only used when addressing someone.