My guess- in a situation like yours where you own 2 houses -is that you must nominate one as your principal private residence. No CGT would be payable on a disposal of that. The other house would be treated as an investment property.
I think (and this is something I'd say you need to clarify) the fact that you retained the Dublin house instead of selling it when you moved to the country automatically 'transformed' it into an investment property at that date- or that's how I'd guess the Rev Commsrs will view it. Now that you've sold it, CGT will be due on the difference between the market value at that date (which should be higher than the original purchase price of it given the way house prices are increasing) and the sales proceeds.
I'm not sure but I don't think it's possible to use the "necessitated by employment" rule to try avoid CGT on the sale of your investment property. That would only apply to the sale of your PPR as - in my understanding- this rule allows you not to occupy your PPR for periods (due to employment situation) and avoid CGT on the sale of the PPR- this is NOT what you're doing, as you are selling the property which isn't your PPR.
Still, it's worth a shot checking it out professionally.