"They were having a drink too!"
Translation:Bha iadsan a' gabhail deoch cuideachd!
Grammatically iad would be OK, but looking at the sentence using the emphatic iadsan is more natural. That’s because they were having a drink too! suggests a direct comparison – stop scolding just us, that’s unfair, they are also at fault, they were having a drink too! or maybe we weren’t selfish and didn’t drink everything ourselves – they were having a drink too – and emphatic pronouns are very often used in such comparisons, to show the change of focus from one person to another.
See also something like:
‘Dè an t-ainm a th’ ort?’ (What is your name?)
‘Seumas a th’ ormsa. Agus dè an t-ainm a th’ ortsa?’ (James is my name. And what’s your name?)
‘An robh thu ann an Albainn a-riamh?’ (Have you ever been to Scotland?)
‘Bha. An robh thusa an-sin?’ (I have. Have you been there?)
‘A bheil cat agad?’ (Do you have a cat?)
‘Chan eil. Ach tha cat aicese.’ (I don’t. But she has a cat.)
But it is just an emphasis for change-of-focus. As I said, grammatically both iad and iadsan are valid.