It will particularly hit Connemara and Mayo, but the rest of the county will get less rain'.
A spell of strong winds is expected, initially mainly in the far southwest of England and across west and south Wales.
Up to 40mm of rain can be expected in some areas, with a risk of local flooding.
The warning is from 21.00 on Monday 17 September until 18.00 the following day.
A Met Éireann forecaster said: 'Ahead of the storm there will be heavy rain in the west.
It will be mostly cloudy elsewhere with just scattered outbreaks of rain and drizzle, but with limited bright or sunny spells will shine through also. Close and humid with highs of 17C to 19C.
According to Met Eireann, current forecasts indicate that south to southeast winds will reach gale or strong gale force at Sea tomorrow, but will be fresh to strong over land.
Helene is now one of a glut of tropical storms in the Atlantic, with Florence sparking mass evacuations as it heads towards landfall in the USA states of North Carolina, South Carolina and Virginia.
A full yellow warning is in place for north east England, north west England, Northern Ireland, southwest Scotland and Lothian Borders, south west England, Strathclyde, Wales and Yorkshire & Humber.
Tomorrow will start off windy, with temperatures of between 16C and 20C.
"Further rain overnight, heavy and persistent at times".
The UK is still set to bathe in the high teens and early twenties for the rest of the week. It is classified as a strong category 2 hurricane with wind speeds of 150 kilometers per hour.