In an appeal, gardai say the serious injury collision happened on the R551 at Caherslee, Tralee, County, Kerry shortly before 1.30am on Saturday.
"The cyclist, aged in his 50s, was conveyed to University Hospital Kerry where he remains in a serious condition. No other injuries were reported at the time," said a garda spokesperson.
The road when the incident occurred remains closed for technical examination, with local diversions in place and gardaí are appealing for any witnesses to this collision to come forward.
"Any road users who may have camera footage, including dash-cam, and who were travelling in the Caherslee area of Tralee in the early hours of Saturday are asked to make this footage available to investigating gardaí.
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Anyone with any information should contact Tralee garda station on (066) 7102300, the Garda Confidential Line on 1800 666 111, or any garda station.
Investigations are ongoing.
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