Good suggestion, Annraoí. Have a wee look here

I'm also sure that a library could help you out

Or a good old search at Google could do the trick as well.

The thing about this subject, with all due respect, is that there is too much to explain, and translations can be ambiguous, as can interpretations of translations.
When I first saw your post, Mudguard, I said to myself: "Where do we start". That would be my reading of it.

For instance, I come from Portadown - County Armagh.

Portadown can be translated two ways;
Port an Dúnain - meaning 'The point of the fort'. This alludes to the fact that in the 1500's and onwards there was a British fort at what they call 'The Point of Whitecoat"

Port an dhá Ábhainn - meaning 'The point of the two rivers'.

The two places are actually the one, because 'The Point of Whitecoat' was also the place where the Bann River met the Cusher, 2 miles south of the town. The Newry Canal was later joined to this confluence.

But which one is the right one?

It's a HUGE topic, and one man's definition might not necessarily be the proper one, and would only serve to confuse you more.

Check out those links, go to a library; it really is a fascinating subject, and you'll find that a lot of the names of places have roots in something very local, their geography, for example, which make general translations innappropriate. Here's a good one, a townland outside of Portadown:
Knocknamuckley.

Anyone want to have a go?

B.