The Fencibles, 1799-1803.
Consisted of infantry and cavalry units, locally raised and controlled in response to specific threats.

The Volunteers, 1803-1813.
Raised by the July 1803 Levy en Masse Act, locally raised and controlled but publicly funded units, with the rank and file committed to 24 days of military training per annum. Due to problems of enforcing command structures and spiraling costs it was decided in 1808 to integrate this unit into a Local Militia, however, many units refused to disband and remained independent of regular military control. In 1813, the Volunteers numbered approximately 68,000 all ranks.

The Local Militia.
Raised in July 1808 in response to the problems posed by the Volunteers. Attempted to assert more control on locally raised units. In 1813 the Local Militia numbered approximately 304,000 all ranks.

The Militia.
Established by the 1757 Militia Act. Service was voluntary and for five years, as opposed to regular enlistment which until 1806 was for life; however, if counties were unable to find enough recruits for their Militia units, they augmented their numbers by a ballot of the local parishes. The Militia units were predominantly based in England and Wales, as its policing role led it to being viewed by the majority of Irish as a tool of English and/or Protestant opprression, particularly in light if its prominent role in the suppression of the 1798 Irish Rebellion. Hence, in 1802, the Militia units in Ireland were disbanded due to problems with recruitment. In 1813, the Militia numbered approximately 93,000 all ranks.

From:  The Armies of Wellington by P.J. Haythornthwaite.