"Irish is being spoken by them."
Translation:Tá an Ghaeilge á labhairt acu.
I don't think Gaeilge should be definite here. Graiméar Gaeilge na mBráithre Críostaí mentions in section 7.14 that the article is only used before language names when they're used with broad meaning. It then gives some examples like Abair i mBéarla é and Tá Gréigis aige where the article is not used. The sentence in question here definitely fits better into this latter category.