is he visa exempt? if he's visa exempt, then he doesn't have to send his physical passport anywhere. a lot of visa exempt applicants are waived ppr and just receive their approval paper. it takes as long as it takes for the letter to arrive from the visa office in regular mail. no one can tell you how long it will take.
if he requires a visa, then you will want to ask in the london thread about whether he will receive an email or a mailed letter with instructions about how to send his passport to the visa office for processing. again, no one can really tell you how long it will take to receive this letter. the london thread has very good statistics for how long it generally takes and what to expect, so it's worth taking a look at the spreadsheet to figure out what happens next.
i do not know if irish applicants are visa exempt or not. i would assume you and your husband would know this information.