I think Scafell Pike (highest mountain in England) might be the most ideal point to see the Republic of Ireland, as it's close to the West coast, and has a westward view not obstructed by mountainous Wales. From what I can see, you'd need to build an observation tower about 410m tall on Scafell Pike to see Irish land (specifically Slieve Foye): https://www.udeuschle.de/panoramas/panqueryfull.aspx?mode=ne...