My mother has a paper dictionary with the IPA pronunciation of very many placenames in the UK & Ireland. I think it's from the 1960s and she bought it when she studied English at university in The Netherlands. Is there an extensive online resource describing the pronunciation of placenames in the UK & Ireland?
For example, for Lough Beagh, I'm guessing something along the lines of /ˌlɒx ˈbeɪ/ in analogy with Lough Neagh, but analogies are an unreliable guide in placename pronunciation (in particular since Lough Beagh is apparently also known as Lough Veagh), so one really does need a complete list. Googling gives some results which appear untrustworthy; for example, pronouncenames.com says it is pronounced /nay/ but I find that hard to believe; I've never heard an open front unrounded vowel followed by a close front rounded vowel and it's very different from /'beɪ/, but perhaps pronouncenames.com by "nay" means there is no pronunciation available. Otherwise Google mostly lists articles with "X British place names hardest to pronounce" etc.
(Of course, many place names, including Lough Beagh, have Gaelic origins, which does not make it easier to figure out the pronunciation)