As a Dub living 30 years in Limerick i have tracked property prices in the two cities for much of my time in Limerick. In general, Dublin prices were 50% dearer than Limerick house prices, except in the nauties boom and currently.
In my view Limerick house prices, in fact prices in most cites (except Galway), are good value currently. An Irish mate of mine who lives in Luxembourg agrees with this view and recently purchased 4 properties in Ireland, all outside Dublin. So without Brexit impact I would say buy - in fact I think prices will continue to go up in Limerick as the job market down here is very positive and looks like it will remain so for some time.
Your question was around the impact of Brexit. I suspect any Brexit outcome will take time to flow through to house prices. One concern being expressed by industry CEO's here in Limerick is that a lot of their product is shipped to the EU market via land-bridge across the UK. If UK hard brexits this could be very problematic for a lot of Irish manufacturing operations. The entire Food industry here is also very exposed as UK is our main market.
If you are really concerned then a simple solution is to rent and await the Brexit outcome, as we are only 5-6 months away.