By Padraic Halpin DUBLIN (Reuters) – Sinn Fein leader Mary Lou McDonald on Wednesday said Britain’s proposals to override some post-Brexit trade rules in Northern Ireland were “astonishing” and yet another move by London that only serves to boost the Irish nationalist party’s quest for a united Ireland. British Foreign Secretary Liz Truss set out steps on Tuesday to try to break a deadlock with the European Union on the Northern Ireland protocol, lining up a new law to unilaterally ease the movement of goods if talks with Brussels fail. The move further inflamed relations with the EU, includin…