Thanks for your question. I will try to help.
Unfortunately, you have to submit originals. The last time I checked they did not accept certified copies.
They don’t really justify their requirements, they just state what they are and then it’s for applicants to comply with. They are very intransigent about it.
You may consider sending both your originals and certified copies of your passport and asking in your covering letter for them to exercise discretion and consider examining the original passport first and then retaining the certified copies for their file. You can state that if they are not minded to do this then you want the application and documents in entirety returned to you and see if this works.
I expect they will require you submit both passports, but again you can consider asking for one to be returned to you.
However, I would not travel outside of the UK on your Canadian passport because you will need to re-enter the UK and your Canadian passport would not have a visa on it. So, the home office entry clearance officers would be likely to refuse entry because you would not be able to produce evidence of your Irish nationality because your Irish passport/ID card would be with the home office
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