Why the Irish Sea lay beyond the main E-boat threat

Were motor torpedo boats needed in the Irish Sea because E-boats threatened places such as Pembrokeshire?

Not to the same degree, because the Irish Sea was generally too far from German bases for the E-boats' practical operating range.

E-boats did not normally reach the Irish Sea in the way they operated in the Channel. Published range figures could vary, but the distance from German operating bases made Pembrokeshire and the western approaches a different problem. Sea state and the need to retain fuel for combat and return also reduced what a headline range number meant in practice. Threat geography therefore determined where scarce coastal craft were most useful.