Learn French: On a découvert des ruines il y a un an. - We discovered some ruins a year ago.

Yes! Normally, il y a means "there is" or "there are". However, when you put a duration of time right after it, like un an (a year), it changes meaning to "ago". So il y a un an translates perfectly to "a year ago".

Why is on translated as "we" instead of "one"?

In everyday spoken French, people almost always use on to mean "we" instead of using the formal nous. Just remember that even when on means "we", the verb is still conjugated in the singular form, exactly as it is for il or elle.

Is the past participle découvert irregular?

Yes. The infinitive verb is découvrir (to discover). It is an irregular verb in the past tense, but it follows the exact same pattern as ouvrir (to open), which becomes ouvert.

Can I just say "On a découvert ruines" like in English?

No, you cannot skip the article in French like we often do in English. Because you discovered an unspecified amount of plural ruins, you must use the plural word for "some", which is des.