Tax as a dual citizen

(With the exception of US citizenship), citizenship is rarely relevant to tax liabilties. Revenue authorities care about where you are resident, where you are ordinarily resident, where you are domiciled. They mostly do not care what passport(s) you hold; that's not a tax event.

You need to check out the Ireland/Canada double taxation agreement. Under most DTAs, as a rough rule of thumb, employment income is taxed in the country where the employment is exercised, so your Canadian job would be taxed in Canada, and your Irish job would be taxed in Ireland. But if both countries regard you as a resident (and you'll need to check the definition of "resident" in the DTA) then each would seek to tax you also on your "foreign" employment, but giving you credit for the tax already paid to the foreign revenue authority.

The upshot is that you'll probably pay the higher of tax at Irish rates, and tax at Canadian rates, on all your employment income.

Social security is a whole other ball game. You'll pay Canadian social security on your Canadian earnings, and Irish social insurance on your Irish earnings, and you'll end up with an incomplete contribution record in both countries. Not sure what your optimal strategy here is.