PD,
I have read through the page containing Irenaeus' mention of Mark 16:19. If I am honest I'd say i had a brief moment before I read the whole chapter, where I thought it could have been edited in, but re-reading it, it certainly looks like it fits the context well if you understand what he was writing about.
Of course you're convinced Spud - you will be convinced by anything that appears to back up your pre-judged faith based position, and unconvinced by anything that counters you're pre-judged faith based position, regardless of the evidence.
So let's ask a couple of questions.
1. I presume when you are talking about 'fitting with the context' you are basing this on the original Greek text, perhaps from 200CE ... oops, bit of an issue - there are no versions of that part of Against Heresies in the original Greek and indeed only tiny fragment of any part of the text in Greek, so very difficult to assess alignment with original context.
2. So I guess when you are talking about 'fitting with the context' you are considering the Latin translations that appeared in the late 4thC at pretty well the same time as the overall orthodoxy of christianity was being embedded. And of course a point of translation is the perfect time for tweaking, adding, editing of a text without it appearing completely clunky as the very process of translation takes the translated text away from the original. Maybe I'm wrong, but I suspect you aren't basing your comments on the later Latin translation.
3. So perhaps you are basing your comment on an English translation of a Latin translation of an original Greek text, with the original of that section completely unavailable to us.
So realistically all you can say is that a translation produced by the people responsible for the embedding of orthodoxy in the church at the time it was embedded seems to fit with the agreed orthodox view - no shit Sherlock.
Come on Spud - get real - victors rewrite history in the manner that suits their agenda. So it is hardly surprising that when those who saw themselves as diligent custodians of a church orthodoxy that had been hard won through a couple of centuries of debate and battle end up ensuring that texts, purportedly from smack in the middle of that battle end up seeming to fit with their orthodox view.
Bottom line - we don't know what Irenaeus originally wrote - what we do know is what writings were attributed to Irenaeus by later translators and custodians of that text who had a very clear agenda, and included plenty of serial interpolators or earlier texts - step forward Eusebius.