Here's the song in reference: https://www.youtube.com/watch?v=4Sje2VYw99A
I miswrote the title, in context the warriors are coming to aid the Irish, not to invade.
IIRC didn't the Irish harbor some of the marooned Spanish sailors from the failed Spanish invasion of England?
There are several versions of this song--and French & Spanish have different roles in each!
In the Jacobite version here (18th c.), the line you're thinking of is something like "They'll be with him, Frenchmen and Spaniards." The (Catholic) French and Spanish supported the Jacobite 'Bonnie' Prince Charles against Protestants in the House of Hanover. Part of this support came in the form of Irish and Scottish troops that were in French service.
In 1914, Pádraig Pearse reworked the words and lyrics as a rebel song. To bolster its nationalist sentiment, the Scottish Prince Charles was replaced by the Irish Gráinne Mhaol (Grace O'Mally), and the line about joining with the Spanish and French was replaced with "They're Irish--Not French or Spanish" to emphasize the desire for an independent Irish republic.
Finally, some people (mis)translate "Ag fógairt fáin ar Ghalla" as "They will rout the French", though Ghalla is actually 'foreigners' and not 'Gauls'.