"Seann sgoil."

Translation: An old school.

[pi5zZA0z]

This also sounds like "Seo an sgoil" - to me, anyway. What is the difference I should be hearing?

[silmeth]

seann is pronounced /ʃãũN/ – with the diphthong /au/ and possible nasalization (but I believe not all speakers do that) – the important thing is that there’s the vowel /a/ there and short /u/ right after it. Seo is /ʃɔ/ – the vowel is different, it’s a rounded /ɔ/, and there is no /u/ at the end.

So… seann /ʃãũN/ and seo an… /ʃɔ (ə)N/ are similar (they have the same consonants) but the vowels in them are different.