4.11 The initial consonant is lenited when the noun or the verbal noun is governed by the preposition thar: ní fiú thar phingin é; níor fhan sé thar bhliain ann; ní aithneodh sé cat thar chóiste; thar dhuine [eile]; ní raibh thar chúigear acu ann; thar Chorcaigh; thar mholadh beirte; thar dhroichead na habhann; thar chlaí an bhóthair; thar bharr an chnoic; thar Shliabh an Iolair.

The noun is not lenited after thar in certain phrases that involve unqualified indefinite nouns and that have a general meaning: thar barr (amach); thar bóchna; thar bráid; thar cionn; thar cnoc (soir); thar droichead; thar fóir; thar muir; thar sáile; thar sliabh; thar tír (amach); thar toinn.

The second particle of the compound preposition thar ceann is not lenited: chuaigh an bhanaltra thar ceann an dochtúra; but chuaigh an piléar thar cheann an dochtúra.

From Graiméar Gaeilge na mBráithre Críostaí (An Gúm, 1999)