Heuston to city centre should be fine, too far on the other side of that might be a little ropey.
Never actually been to Comedy Crunch and dont know much about it (left Dub in 2013 so it might have more about it now). Been the International Comedy Club (small venue, bit raw but not bad, about 10 euros) and Laughter Lounge ( bigger venue, more professional set up but more expensive). Both are central. Best thing to do is probably find out who is playing on the particular night you want to go and youtube them.
In all honesty, central Dublin is good for drinking but doesnt have a massive amount to do otherwise bar pottering around. Depending on how long you have, you could also take a trip on the DART (local train line) to either Howth or Malahide which are two nice seaside surburbs north of Dublin. Or a day trip to Glendalough in Wicklow to see a more traditional Ireland (lakes, mountains, lots of green albeit a bit touristy).