If the flat was your PPR and you moved away for work reasons, then 4 years of absence count as if it was still your PPR for CGT purposes, but only provided you "re-occupy" it after the absence. So if you claim to "re-occupy" it before selling it, you get 4 years' absence counting as period of owner-occupation.
Regardless of re-occupation, the last three years of ownership will count as owner-occupation for CGT purposes.
So the taxable gain is reduce proportionately. After that, the first 10,100 pounds of profit is exempt from CGT, and the rest is taxed at 18%, provided it doesn't take you to higher rate tax in the UK. As presumably you don't have much income in the UK, your taxable gain, if any, is unlikely to take you over about 43,000 pounds or so.
Also when calculating your gain, you get to deduct improvement costs (if any) and buying and selling expenses, including stamp duty.
My point is your taxable gain, after the above deductions, is likely to be very small or even non-existent, though it depends on your flat, of course, and how much it increased in value.
Not sure, whether you can claim your flat as your PPR throughout the period of ownership, as it's in the different country from where you live. If you were still living in the UK, just in a different place but still stayed at the flat now and again, you could have made an election to the British Revenue to say that this flat was to be considered as your PPR. This election can be made up to 2 years later, so you could elect now for the period beginning February 2009.
Then the period July 2004 to July 2008 would be covered under move away for work rule, and from February 2009 to date of sale under the election. Leaving you with only a fraction of the gain, part of which would be reduced further still under the last 3 years ownership rule. The remaining gain would likely be very small and below 10100 pound annual exemption limit.
It may be worth you checking with an accountant to see whether you can make this election even though you work in Ireland.
Even without the election, you can reduce the gain considerable under 4 years away for work (but need to "re-occupy" the flat, even for a short time) and last 3 years of ownership rule. Then the annual exemption will reduce the gain further or even eliminate it.
Do your figures to see what the gain is likely to be. Then decide if it's worth looking into making this election or "re-occupying" the flat.
Also if you are married, you can transfer the flat into joint ownership with your spouse before selling it, to halve the gain to each of you, to use two annual exemption limits of 10100 each.