Well as an Irish citizen, at least, you don't need a visa, but still need the ETA (do it online to prevent delays).
Most visitors are admitted for six months, unless otherwise specified by the Border Officer at time of entry.
How long before your next trip? Depends on the border officer's view of your plans. Having a house will raise flags. He'll wonder if you're trying to 'live' in Canada without really living there (travelling in and out repeatedly). So you may be required to provide evidence that you live in the US - bank accounts, utilities bill, contract of employment etc, and show the date you anticipate leaving Canada, possibly even evidence of your return ticket.
(sources include those linked, and my personal experience after living there for a year, and several other trips)