The Time for isha Prayer is after Twilight Disappears. But this will not happen during winter months for Cities near the poles, which is the case for Dublin now.
Coming to How to pray in such situation, Authentic
his Hadith from IbnMajah where Prophet Mohammed (peace be upon him) said link
.............. We said: 'O Messenger of Allah, how long will he stay
on earth?' He said: 'Forty days, one day like a year, one day like a
month, one day like a week, and the rest of his days like your days.'
We said: 'O Messenger of Allah, on that day which is like a year, will
the prayers of one day suffice us?' He said: 'Make an estimate of time
(and then observe prayer).'............
This Hadith clearly says that five prayers every 24 Hrs even if sun doesn't set. When we have a direct reference from Prophet Mohammed (peace be upon him), then there is no wrong in estimating.
Based on the graph here, both of the time seems to be an estimate (I am not an expert on this).
So in your case, You can follow any of these. But stick with one always. Aside from this, It is A Sunnah to pray in Congregation.