locals will be better able to comment.
Here goes! I'm a 'local'... born and bred in Mullingar - so beware, I might be slightly biased. Note the following is solely my opinions from living in the town for the best part of 20yrs.
Regarding top/bottom areas:
The traditionally most sought after areas in the town would be
Brookfield, Pettiswood, Dublin Road and the
Lynn area.
Traditionally areas seen as having lots of anti-social activity would be
Grange,
Dalton Park and
Springfield.
Naturally you'll find prices really reflect this (€150k vs €450k!)
Other estates:
College Hill - very good area near Brookfield, but a good €100k cheaper (smaller houses/gardens)
Greenpark - too far to the West of the town for commuters... aside from that nothing actually wrong with it.
Ard Leigh/ArdliVale/Ardilaun/Ard XXX - a hugh series of houseing estates with no facilities and cut up by a busy ring road - very affordable, full of commuters... but no facilities
Lakepoint - another huge commuter estate, you'll get lost here... again no proper facilities. Rumors of poor build abound also, particularly in the apartments
Clonmore - I come from near here and wouldnt recommend it... houses are old and anti-social behaviour on the rise.
Marlinstown - perhaps the easiest location to get on the motorway... although i've heard rumors of shoddy builds.
Ashfield - near town with tesco and shops beside it not too far from motorway, reasonably affordable
Of course there are many many more estates, these are just some of the larger ones. Depending on your budget, College Hill, Brookfield, Pettiswood and perhaps a few of the smaller estates would be my personal preference.
If you can find a house you like and can afford, Mullingar offers plenty of good facilities and theres big plans already in motion to add more - the Robinstown Development Plan will eventually see a state of the art sports complex built alongside whole new streets and a larger county grounds. The Lynn Shopping Development already approved is likely to bring Argos/PC World and others to the town. In addition the Blackhall Development (similar in scope to the new shopping center in Athlone) will hopefully get through the planning stages soon - it will bring a huge amount of retail and cultural space to the town center. A local bus service (despite many false dawns) is likely to get the go ahead in 08/09.
With a population arguably bigger than Athlone, Sligo, or Kilkenny - and certainly bigger than Portlaois, the town will hopefully seen continued growth in the coming years... and hopefully for my sake it'll become an employment center in its own right soon!!
I know the above might sound like marketing
speil from an over enthusiastic local - but as I warned you I might be biased! Feel free to ask me any questions if you think I can be of use.