I had a tooth extracted and a abcess found afterwards. the dentist did no irrigation of the wound and gave me no antibiotics, should l be concerned? also shouldn't the abcess have shown up on the xray prior to extraction?? thanks?

Yes should have shown up if X-ray taken at best angle. Drainage if good may clear abscess with out antibiotics. Old adage: evil puss! Let it out!

The dentist did not give you antibiotics since at the time of the extraction s/he did not feel you had an infection... as per your post the infection was discovered afterwards. Regarding radiographs, it's not a perfect world... they don't always identify every possible pathologic lesion. If you're healing well now, no concerns.