Getting *to* Heathrow from Ireland is fine, and the tube & Heathrow Express provide good connections. The problem can be getting back out of Heathrow at 1730 on a weekday evening when the queue to get through security can be huge (and unmanaged). I don't know what the other airports are like, but because you share a queue with all the UK domestic flights from Heathrow there can be a lot of people doing the evening commute by plane there.
I would consider using different airports for getting there and back as necessary, especially if you will be going to work/college like it sounds. If you plan on coming back after a hard weeks work the last thing you would want to do is to have to make a few tube/train connections to get to the far side of London just to save €30 on the fare or because the timing for the outbound flight was the best. Getting across London, especially where it involves connections can soak up a huge amount of time, and when you are on your way home for a short weekend you might find the time is better spent with your friends in Cork than sitting on the tube with strangers.
Summary - examine all options, bearing in mind that these will be affected by such things as whether you are flying in on Sunday evening or Monday morning, and again whether you are flying out Friday rush hour, or earlier or later in the day.
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