Northern property has been extremely bouyant these past few years - a lot of that was bringing it into line with the rest of the UK. How much more there is left I don't know, but maybe not as much as other's think. Every man and his dog is currently an investor, which leads one to think that certainly the bargains are long gone, you have to look at rental yield and the north's economy as a whole.
Rent is generally paid by the DHSS who are keen to transfer the provision of housing entirely to the private sector. However they don't pay a lot - from my knowledge, a 3 bedroom semi will get about £400 a month (at least in a provincial town a little smaller in Derry, not sure if they adjust the rent for the location - they may not).
As you know Derry you'll know what the rental market is like there anyway.
Re the North as a whole the biggest problem is over-reliance on the public sector, something like 40% of the economy is dependant on this, one of the bank chairmen was commenting on that as a problem last week.
For mortgage, I don't know that you can get a mortgage on a property outside of this jurisdiction, possibly, but it may just be on equity in your own property. Interest rates are obviously lower here, but then you'd be exposed to forex fluctuations.
CGT would b payable here at the normal rate, income tax likewise - you'd have to check the double tax agreement re credits given between here and UK on rental income, also you'd have to ascertain the situation with regards deductability of mortgage interest from UK rental property etc.
In general, whilst I would'nt put someone off the idea, i would caution that imo the best deals are long gone and it's up to you decide if the area you have your eye on represents value (please note that i'm also very bearish on free state property at the minute, so i'm probably a bit biased!!) Good luck.