It has been said that the Irish speak English better than any other race, better even than the British. But the English spoken in Ireland is different to what they speak in Britain or elsewhere. In some areas you might not notice. When they speak amongst themselves, however, you might have more difficulty. The accent is only part of the story.
Accents vary from place to place in any country and Ireland is no different.
You say tomayto and they say tomahto , but you can still understand what they say. However, some of the Irish accents are quite difficult. Part of the problem is speed. The Irish generally speak quickly and it can be difficult to keep up. In Dublin it is very common to drop the final t in words such as what, at, brat, sweet and treat.