A Spanish plume is a warm wind originating over central areas of Spain moving north towards Ireland on a southerly flow. This wind can bring some very high temperatures but can also bring heavy rainfall and thunderstorms. Most of the time it affects the UK and France.
For it to affect Ireland, we would need an approaching low pressure system and it's fronts to stay to the west of the Bay of Biscay or to the southwest of Ireland and stall with a high over central Europe. This will draw the winds up on a south to southeast flow and will turn very warm and humid. The wind is sourced from Spain and south France and is forced to ascend by the Pyrenees mtns. This wind is also aided in it's ascent by a hot and drier air from the continent. As it moves out into the Bay of Biscay, the plume is elevated over cooler waters as colder air is much denser and hugs the surface. Along the elevated airmass, the air destabilises and thunderstorms can rapidly develop over the Bay of Biscay. A cold front from the parent Low pressure to our south west can force the air to ascend ahead of the boundary that thunderstorms can break out overland in warm and humid airmass. With winds continuing from a south to southeast direction, these storms approach Ireland and can give significant rains and frequent lightning. Overland, the air is can be so warm that a thunderstorm is ongoing but no rain hitting the grround, this is called a dry storm but the air at the surface is that warm that the rain can evaporate before reaching the surafce. This is called Virga.
This occurred on the 25th of August 2000 when thunderstorms broke out over Leinster during the day in a Spanish Plume but it wasn't until later that evening when storms gained strength and frequent lightning affected the eastern side of Ireland with downpours but dry spells even though lightning was ongoing overhead.
Sometimes, Spanish plumes have originated further south into Africa where dust particles in the atmosphere have deposited in and around Ireland in rainfall after getting caught up in the air flow.
The charts below shows a rough guide to what a plume may look like with thundery breakdown from Biscay.