Electoral law provides that a ballot paper at an election is invalid for a number of reasons: if it does not bear the official mark; does not clearly indicate a first preference for some candidate; indicates a first preference for more than one candidate; or has anything written or marked on it, which in the opinion of the returning officer, is calculated to identify the elector.
The results of the general election held on 8 February 2020 are available on the Oireachtas website at:
https://www.oireachtas.ie/en/publications/?q=&date=&term=%2Fie%2Foireachtas%2Fhouse%2Fdail%2F33&fromDate=01%2F05%2F2020&toDate=01%2F05%2F2020&topic%5B%5D=elections
Table 4 of the results book includes, by constituency, an analysis of the number of invalid ballot papers, including those that did not bear the official mark.