Clondalkin does seem the best interpretation of 'Clankalkin', and it's both a civil parish and also had a Catholic Chapel which was part of the parish of 'Palmerstown, Lucan, and Clondaklin'. The only other similar civil parishes, i.e. starting with 'clon' or 'clan' I can think of are Clonmethan and Clonsilla, and the only other town which includes a 'k' is Clonskeagh, but that is not a Catholic or Civil/Church of Ireland parish - but part of Donnybrook civil parish. One problem is that there are quite a number of parishes in Co. Dublin that do not have records back as far as 1815.
Catholic records for Clondalkin parish were filmed by the National Library and for Clondalkin parish cover back to 1778, although there may be gaps. For register images see Clondalkin, Lucan & Palmerstown See also the related registers of Palmerstown. Records have also been transcibed and indexed for both these parishes on the free IrishGenealogy website Clondalkin & Palmerstown. There are a number of Donnolly records in these, nothing particularly promising that I could find - best to search with wildcards to cope with spelling variation e.g. don*l*y*. Firstnames can appear in Catholic registers in Latin so John may appear as Johannes etc
Do you know what denomination your Donnelly family were ?
Church of Ireland records for Clondalkin Parish are also on the IrishGenealogy website - see here.