The Trekker covers all of the Republic of Ireland (as far as Dundalk) and is valid for four consecutive days from date of issue and costs €110 (£87), and like every other rail ranger / rover ticket on the island has no time restrictions.
iLink in Northern Ireland is £15.50 per day for the whole country (both rail and bus) or £57 for one week. There are four zone radiating from Belfast, naturally the more expensive for the more you cover.
And a piece of advice too - if you do Dublin-Sligo, make sure you take a book or something. Maynooth-Mullingar, especially from Enfield onwards, is the dullest line I have ever done in my life. The rest of it isn't breathtaking either sadly.