DUBLIN, Jan 5 (Reuters) - Growth in Ireland"s service sector slowed sharply for the second successive month in December as firms reported that a rising wave of COVID-19 infections was dampening demand, a survey found on Wednesday. The AIB IHS Markit Purchasing Managers" Index (PMI) dropped to 55.4 from 59.3 in November and 63.4 in October. The index hit a nearly 21-year high of 66.6 in July when most curbs to slow the spread of COVID-19 were removed. Ireland tightened restrictions again last month, shutting nightclubs and ordering pubs and restaurants to close at 8 p.m.. The transport, tourism and leisure sector suffered its first marginal decline in activity since March as a result, the PMI data showed. Overall activity nevertheless remained well ahead of the 50 mark that separates growth from contraction and the deep declines of previous full lockdowns. The tenth successive month of growth in December was also slightly above the long-run survey average. Cost pressures remained severe in December, the survey"s authors said. The rate of inflation eased, but was among the sharpest on record with a similar pattern emerging in prices charged. The 12-month outlook picked up slightly though, with around 56% of firms expecting growth in their businesses by the end of 2022, compared with 51% in November.