You will need to check out the Canada-Ireland tax treaty to see how rental income is to be handled. Ie is tax due at source. In the country where the income arises, or is it up to the receiving end to tax the income. The tax treaty is designed to avoid double taxation.

If the treaty states taxed at source, you can claim exemption from CRA tax on that income.
If the treaty states taxed on receipt, you can claim a Foreign Tax Credit for tax paid in Ireland.

Either way, for 2015 and earlier years you will need to file adjustments (T1-Adj.) for each year in question. Depending on the amount of money involved, you might be advised to get an accountant to help you come up to speed.