Apparently it's pronounced "quill"
From : http://www.pcworld.com/businesscenter/article/149167/whats_in_a_name_better_not_ask_cuil.html
"That explanation is new to Foras na Gaeilge, the group that is essentially the official keeper of the Irish language, responsible for promoting use of the language as well as developing dictionaries and new terminologies. "I am unaware myself of the meaning 'knowledge' being with the word 'cuil' in Irish," Stiofán Ó Deoráin, an official on Foras na Gaeilge's terminology committee, said via e-mail.
He did caution that accents can make a big difference in Irish. In fact, cuil should have an accent on the "u" if it is to be pronounced "cool," like the company says it should. Cuil, without an accent, should be pronounced like "quill," Ó Deoráin said. His point was that the accent, in addition to changing the pronunciations, can change the meaning of a word."