There’s a relief! An agreement concluded between the UK and Irish governments this month (February 2019) reaffirms that existing cross-border social security benefits, including the UK state pension among other things, will continue after Brexit. This agreement was concluded under the auspices of the CTA (Common Travel Area) regulations so, it uniquely applies to Irish and British citizens. It does not address British nationals living and working in the EU and reciprocal rights for EU nationals living and working in the UK.
This really is very good news because, up to this agreement, continued access to the respective social security benefits for both the British and Irish was in doubt.
Also, see the press announcement of the bilateral deal with the two governments in the Irish Independent, 20 Feb 2019.