If you want to spend some good time hiking, Wicklow is one of the most beautiful location not too far away from Dublin, particulary if you are a stranger and you've never visit Ireland and experienced its exceptional landscape of this Country.
It is a little bit difficult to reach Wicklow if you don't have a car. In this case you maybe have a look at what buseireann has to offer.
I'm prety new to Ireland and I'm sure that there are differnt ways to reach this place, but this can be a start point (and every suggestion is welcomed) nevertheless.
In Wicklow you can find a good number of different mountain trails for every taste and pace. The best thing is that you can come across different landscapes as lakes, mountains, forests, fields. So you can't get bored!
Just remember to check the Irish wather!