Would the UK want a hard border? Otherwise any EU citizen could travel to Ireland. Walk across the border, and hence enter UK without any checks. If you get deported from the UK, just walk back across the border. etc. Would the UK accept that?
Yes, but remember: Ireland is not part of the Schengen area. EU citizens (along with everyone else) go through border checks when entering Ireland from outside the common travel area.
That information may be shared with the UK as part of the common travel area agreement, so it doesn't make a difference whether someone enters the common travel area in Ireland or in the UK.
And of course, being allowed to enter a country (whether checked or not) does not necessarily mean you have the right to reside there and seek employment.